Ison_objectagg implemented.

post-review fixes:
* test for dependent subqueries
* test for triggers and routines
* disallow INSERT...RETURNING in triggers and stored functions
* don't return anything if INSERT IGNORE ignored an error

when there are nested subqueries, and a field in a subquery is
resolved as an outer reference to a table few levels up, all subqueries
the subquery with a reference and all subqueries up to subquery with
the table must be marked as dependent.

in the text protocol and PS-prepare step it happens in
Item_field::fix_outer_field in a loop that walks contexts
using context->outer_context.

in PS-execute step Item_field->cached_table is set and subqueries
are walked in a PS-only mark_select_range_as_dependent(),
which inconsistently walks SELECT_LEX'es using select_lex->outer_select().

Fix mark_select_range_as_dependent() to walk contexts, not SELECT_LEX'es,
to have the same logic both in prepare and execute steps.

This fixes a crash in main.insert_returning in --ps-protocol

Closes #1384

comments, whitespaces

because internally setup_wild() adjusts select_lex->with_wild directly
anyway, so there is no reason to pretend that the number of '*' may be
anything else but select_lex->with_wild

And don't update select_lex->item_list, because fields can come
from anywhere and don't necessarily have to be copied into select_lex.

remove inconsistent limitation

Show the config file in I_S.SYSTEM_VARIABLES
But only if the user has FILE privilege

change get_one_option() prototype to pass the filename and
not to pass the redundant optid.

make load_defaults() store the file name in the generated option list
using a special marker ---file-marker--- option.

Pick up this filename in handle_options().

Remove ---args-separator---, use ---file-marker--- with an empty file
name instead - this simplifies checks on the caller, only one special
option to recognize.

only my_getopt should use it, because it changes my_getopt's behavior.
If one simply wants to skip the separator - don't ask it to be added
in the first place

process all --defaults* options uniformly,
get rid of special case for --no-defaults and --print-defaults
use realpath instead of blindly concatenating pwd and relative path.

it turns out that practically every single user of handle_options()
used the get_one_option callback. Simplify the code,
make it mandatory, adjust unit tests.

almost all my_getopt settings and callbacks are global variables,
directly assignable to configure my_getopt. Only getopt_get_addr
was using a setter function. Get rid of it, make it a global
directly assignable variable like all other settings.

Also make getopt_compare_strings() static.

This is a remnant of "MySQL Instance Manager", which was removed in
MySQL-5.5.0 and never existed in MariaDB

Remove callback, simplify and optimize the code accordingly.

MDEV-20818 ER_CRASHED_ON_USAGE or Assertion `length <= column->length' failed in write_block_record on temporary table

The patch for `MDEV-20795 CAST(inet6 AS BINARY) returns wrong result`
unintentionally changed what Item_char_typecast::type_handler()
returns. This broke UNIONs with the BINARY() function, as the Aria
engine started to get columns of unexpected data types.

Restoring previous behaviour, to return
  Type_handler::string_type_handler(max_length).

The prototype for Item_handed_func::return_type_handler() has changed
from:
  const Type_handler *return_type_handler() const
to:
  const Type_handler *return_type_handler(const Item_handled_func *) const

Putting "#ifndef NO_EMBEDDED_ACCESS_CHECKS" around these constant declarations:

const uint max_hostname_length= 60;
const uint max_dbname_length= 64;

Suppress warnings when Field_inet6::store() is called from
read_statistics_for_table() and other optimizer related routines.

This patch does for Field_inet6 the same thing with what Monty's
patch previously did for other Field_xxx:

> commit 1bbe8c5e
> Author: Michael Widenius <monty@mariadb.org>
> Date:   Sun Sep 22 04:08:48 2019 +0300
>
>    Proper fix for disabling warnings in read_statistics_for_table().
>    MDEV-20589: Server still crashes in Field::set_warning_truncated_wrong_value

Alas, some meaningful warnings disappeared.

These tests use only HEAP and MyISAM tables.

Adding the "override" keyword to Item_char_typecast_func_handler_inet6_to_binary::type_handler_for_create_select()

Clang fails to compile without it.

Engine get LIMIT/OFFSET info an can it use/reset.

Kill check moved from send_data() methids in its wrapper:
send_data_with_check().

Now both offset and limit are stored and do not chenged during execution
(offset is decreased during processing in versions before 10.5).

(Big part of this changes made by Monty)

rec_init_offsets(): Relax the assertion that was added in
commit 01f45bec
to catch ROW_FORMAT=REDUNDANT records that have fewer fields
than expected.

This assertion would fail when accessing the records of the
built-in InnoDB table SYS_INDEXES. The column MERGE_THRESHOLD
had been effectively instantly added in MariaDB Server 10.2
(and MySQL 5.7), but is_instant() does not hold for that index.
Relax the assertion, so that it will not fail in this case.

MDEV-20813 Assertion failure in buf_flush_init_for_writing() for innodb_immediate_scrub_data_uncompressed=ON

The assertion that was added in
commit c0c003be
to augment the fix of MDEV-20805 turns out to be invalid when
innodb_immediate_scrub_data_uncompressed is enabled.
In this mode, fsp_init_file_page() will be invoked on data pages
that have been freed, causing writes of almost-all-zero pages.

btr_page_free(): Adjust the comment.

buf_flush_init_for_writing(): Disable the assertion with a note
that it should be re-enabled in MDEV-15528.

When the mysqld_multi script passes the --defaults-group-suffix
option to mysqld, it must remove the initial substring with the
group name ("mysqld") from option value, because otherwise substring
"mysqld" will be added to the group name and then the group name
will contain the word "mysqld" twice, which is wrong, because
mysqld itself adds the suffix received to the group name.

When the mysqld_multi script passes the --defaults-group-suffix
option to mysqld, it must remove the initial substring with the
group name ("mysqld") from option value, because otherwise substring
"mysqld" will be added to the group name and then the group name
will contain the word "mysqld" twice, which is wrong, because
mysqld itself adds the suffix received to the group name.