MDEV-7563 Support CHECK constraint as in (or close to) SQL Standard

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

sql/sql_alter.h

@@ -124,6 +124,8 @@ public:
   // Set for ADD [COLUMN] FIRST | AFTER
   static const uint ALTER_COLUMN_ORDER          = 1L << 26;
 
+  static const uint ALTER_ADD_CONSTRAINT        = 1L << 27;
+  static const uint ALTER_DROP_CONSTRAINT       = 1L << 28;
 
   enum enum_enable_or_disable { LEAVE_AS_IS, ENABLE, DISABLE };
 
@@ -172,6 +174,7 @@ public:
   List<Key>                     key_list;
   // List of columns, used by both CREATE and ALTER TABLE.
   List<Create_field>            create_list;
+  List<Virtual_column_info>     constraint_list;
   // Type of ALTER TABLE operation.
   uint                          flags;
   // Enable or disable keys.
@@ -200,6 +203,7 @@ public:
     alter_list.empty();
     key_list.empty();
     create_list.empty();
+    constraint_list.empty();
     flags= 0;
     keys_onoff= LEAVE_AS_IS;
     num_parts= 0;