IFNULL(inet6_not_null_expr, 'foo') erroneously set its nullability to NOT NULL.

Fix:
- Moving the line "maybe_null= args[1]->maybe_null" before the call
  of fix_length_and_dec2(), so the call of Type_handler method
  Item_hybrid_func_fix_attributes() can reset it when desired.

- Fixing Type_handler_inet6::Item_hybrid_func_fix_attributes()
  to ignore args[0] when detecting nullability of IFNULL().

In the merge eae968f6, it turns out that
I had accidentally initiated an in-source build in the past, and
$MYSQL_TZINFO_TO_SQL was pointing to a stale copy of the executable in
the source directory, instead of the correct one in the build directory.

The test timeouts, because mtr is waiting for pid file.
The pid file is not there, because expected to fail mysqld startup
(duplicate named pipe name), this startup removed pid file of the running
mysqld instance.

To fix, split handle_connections_win() into the initialization part,
and  accept/handle part, like it is done elsewhere.

The initialization part runs before pid file handling, and aborts on errors
, thus avoiding pid file overwrites.

The test encryption.create_or_replace would occasionally fail,
because some fil_space_t::n_pending_ops would never be decremented.

fil_crypt_find_space_to_rotate(): If rotate_thread_t::should_shutdown()
holds due to innodb_encryption_threads having been reduced, do
release the reference.

fil_space_remove_from_keyrotation(), fil_space_next(): Declare the
functions static, simplify a little, and define in the same compilation
unit with the only caller, fil_crypt_find_space_to_rotate().

fil_crypt_key_mutex: Remove (unused).

The background DROP TABLE queue may be blocked for some more time
due to MDEV-16678. Let us apply similar adjustments as earlier:
commit 6af00b2c
commit 89633995
commit ccd87d34

Example of the failure:
http://buildbot.askmonty.org/buildbot/builders/bld-p9-rhel7/builds/4417/steps/mtr/logs/stdio
```
main.mysqld--help 'unix'                 w17 [ fail ]
        Test ended at 2020-06-20 18:51:45

CURRENT_TEST: main.mysqld--help
--- /opt/buildbot-slave/bld-p9-rhel7/build/mysql-test/main/mysqld--help.result	2020-06-20 16:06:49.903604179 +0300
+++ /opt/buildbot-slave/bld-p9-rhel7/build/mysql-test/main/mysqld--help.reject	2020-06-20 18:51:44.886766820 +0300
@@ -1797,10 +1797,10 @@
 sync-relay-log-info 10000
 sysdate-is-now FALSE
 system-versioning-alter-history ERROR
-table-cache 421
+table-cache 2000
 table-definition-cache 400
-table-open-cache 421
-table-open-cache-instances 1
+table-open-cache 2000
+table-open-cache-instances 8
 tc-heuristic-recover OFF
 tcp-keepalive-interval 0
 tcp-keepalive-probes 0

mysqltest: Result length mismatch
```
mtr: table_open_cache_basic autosized:

Lets assume that >400 are available and that
we can set the result back to the start value.

All of these system variables are autosized and can
generate MTR output differences.

Closes #1527

MDEV-23415 Server crash or Assertion `dec_length <= str_length' failed in Item_func_format::val_str_ascii

Problem:

The crash happened in FORMAT(double, dec>=31, 'de_DE').

The patch for MDEV-23118 (commit 0041dacc)
did not take into account that String::set_real() has a limit of 31
(FLOATING_POINT_DECIMALS) fractional digits. So for the range of 31..38
digits, set_real() switches to use:
- my_fcvt() - decimal point notation, e.g. 1.9999999999
- my_gcvt() - scientific notation,    e.g. 1e22

my_gcvt() returned a shorter string than Item_func_format::val_str_ascii()
expected to get after the my_fcvt() call, so it crashed on assert.

Solution:

We cannot extend set_real() to use the my_fcvt() mode for the range of
31..38 fractional digits, because set_real() is used in a lot of places
and such a change will break everything.

Introducing String::set_fcvt() which always prints using my_fcvt()
for the whole range of decimals 0..38, supported by the FORMAT() function.

Prevents errors on Windows Linux subsystem.

Fix to pr #1600

null merge (no changes since 5.7.28), update the version

MDEV-23330 Server crash or ASAN negative-size-param in my_strnncollsp_binary / SORT_FIELD_ATTR::compare_packed_varstrings

and
MDEV-23414 Assertion `res->charset() == item->collation.collation' failed in Type_handler_string_result::make_packed_sort_key_part

pack_sort_string() *must* take a collation from the Item, not from the
String value. Because when casting a string to _binary the original
String is not copied for performance reasons, it's reused but its
collation does not match Item's collation anymore.

Note, that String's collation cannot be simply changed to _binary,
because for an Item_string literal the original String must stay
unchanged for the duration of the query.

this partially reverts 61c15ebe

An unfortunate performance regression was introduced in
commit b1ab211d (MDEV-15053)
that affects workloads where the working set is larger than
the buffer pool.

We used to terminate the loop in buf_LRU_scan_and_free_block()
when we succeeded to free a replaceable buffer pool block.
Due to the regression, we would keep freeing all replaceable
blocks until we reach the end of the list or encounter a block
that had never been accessed because it had been brought in
by read-ahead.

for the functionality that was part of the server for years
and is moved in 10.5 into a [statically linked] plugin
without changing the underlying code.

this makes it a weak dependency, signalling that ColumnStore can work
without jemalloc and that while jemalloc is desired, missing jemalloc
is not a fatal error.

this breaks support for Suggests: but we don't use it anyway

partially revert 2c5831b2 - ColumnStore still needs readline to build