it's not enough to look for NOT NULL IS, this also fails queries like
SELECT NOT NULL <=> NULL;
and adds no value anymore, as the grammar now requires parentheses

* remove a confusing method name - Field::set_default_expression()
* remove handler::register_columns_for_write()
* rename stuff
* add asserts
* remove unlikely unlikely
* remove redundant if() conditions
* fix mark_unsupported_function() to report the most important violation
* don't scan vfield list for default values (vfields don't have defaults)
* move handling for DROP CONSTRAINT IF EXIST where it belongs
* don't protect engines from Alter_inplace_info::ALTER_ADD_CONSTRAINT
* comments

make it return same errors on CREATE as

CREATE ... ( ... DEFAULT const ... )

the error should be "subselect is not allowed here", same as for
DEFAULT ((SELECT 1))

and remove all related casts to uchar*
also remove a couple of unused methods

collaterals:
* use %`s, not '%s'
* use correct SQLSTATE codes for these two errors

- Adding SHOW CREATE TABLE into all DEFAULT tests,
  to cover need_parentheses_in_default() for all items
- Fixing a few items not to print parentheses in DEFAULT:
  spatial function-alike predicates, IS_IPV4 and IS_IPV6 functions,
  COLUMN_CHECK() and COLUMN_EXISTS().

(a new rule in sql_yacc.yy)

- Force usage of () around complex DEFAULT expressions
- Give error if DEFAULT expression contains invalid characters
- Don't use const_charset_conversion for stored Item_func_sysconf expressions
  as the result is not constaint over different executions
- Fixed Item_func_user() to not store calculated value in str_value

ENCRYPT(), SHA(), SHA2(),
AES_ENCRYPT(), AES_DECRYPT(),
DES_ENCRYPT(), DES_DECRYPT()

CRC32(), MD5(), FROM_BASE64(), TO_BASE64(), HEX(), UNHEX(),
ENCODE(), DECODE(), PASSWORD(),
COMPRESS(), UNCOMPRESS(), UNCOMPRESSED_LENGTH().

UUID(), UUID_SHORT()

Miscelaneous functions:
BENCHMARK(), SLEEP(), ROW_COUNT(), FOUND_ROWS(),
GET_LOCK(), RELEASE_LOCK(), IS_USED_LOCK(), IS_FREE_LOCK(),
MASTER_POS_WAIT(), MASTER_GTID_WAIT(), BINLOG_GTID_POS(),
ST_GIS_DEBUG(), DECODE_HISTOGRAM(),

Miscelaneous functions:
LIKE_RANGE_MIN(), LIKE_RANGE_MAX(), WEIGHT_STRING(),
GET_FORMAT(), FORMAT(), LOAD_FILE().

Metadata functions

Dynamic column functions

Functions DATABASE() and USER().

INET4/INET6 functions

- INT result functions
- CAST variants: COLLATE, BINARY, CONVERT(..USING)

UDF

Numeric functions with string input

Boolean predicates

XML functions

Adding tests for cast, bit functions, string functions.

GIS functions

Testing non-ASCII string literals.

- real functions
- temporal functions
- hybrid functions

MDEV-10134 Add full support for DEFAULT

- Added support for using tables with MySQL 5.7 virtual fields,
  including MySQL 5.7 syntax
- Better error messages also for old cases
- CREATE ... SELECT now also updates timestamp columns
- Blob can now have default values
- Added new system variable "check_constraint_checks", to turn of
  CHECK constraint checking if needed.
- Removed some engine independent tests in suite vcol to only test myisam
- Moved some tests from 'include' to 't'. Should some day be done for all tests.
- FRM version increased to 11 if one uses virtual fields or constraints
- Changed to use a bitmap to check if a field has got a value, instead of
  setting HAS_EXPLICIT_VALUE bit in field flags
- Expressions can now be up to 65K in total
- Ensure we are not refering to uninitialized fields when handling virtual fields or defaults
- Changed check_vcol_func_processor() to return a bitmap of used types
- Had to change some functions that calculated cached value in fix_fields to do
  this in val() or getdate() instead.
- store_now_in_TIME() now takes a THD argument
- fill_record() now updates default values
- Add a lookahead for NOT NULL, to be able to handle DEFAULT 1+1 NOT NULL
- Automatically generate a name for constraints that doesn't have a name
- Added support for ALTER TABLE DROP CONSTRAINT
- Ensure that partition functions register virtual fields used. This fixes
  some bugs when using virtual fields in a partitioning function