MDEV-34664: Add an option to fix InnoDB's doubling of secondary index cardinalities

(With trivial fixes by sergey@mariadb.com)
Added option fix_innodb_cardinality to optimizer_adjust_secondary_key_costs

Using fix_innodb_cardinality disables the 'divide by 2' of rec_per_key_int
in InnoDB that in effect doubles the Cardinality for secondary keys.
This has the biggest effect for indexes where a few rows has the same key
value. Using this may also cause table scans for very small tables (which
in some cases may be better than an index scan).

The user visible effect is that 'SHOW INDEX FROM table_name' will for
InnoDB show the true Cardinality (and not 2x the real value). It will
also allow the optimizer to chose a better index in some cases as the
division by 2 could have a bad effect for tables with 2-5 identical values
per key.

A few notes about using fix_innodb_cardinality:
- It has direct affect for SHOW INDEX FROM table_name. SHOW INDEX
  will also update the statistics in table share.
- The effect of fix_innodb_cardinality for query plans or EXPLAIN
  is only visible after first open of the table. This is why one must
  do a flush tables or use SHOW INDEX for the option to take effect.
- Using fix_innodb_cardinality can thus affect all user in their query
  plans if they are using the same tables.

Because of this, it is strongly recommended that one uses
optimizer_adjust_secondary_key_costs=fix_innodb_cardinality mainly
in configuration files to not cause issues for other users.

sql/sql_class.h

@@ -214,6 +214,7 @@ extern "C" const char *thd_client_ip(MYSQL_THD thd);
 extern "C" LEX_CSTRING *thd_current_db(MYSQL_THD thd);
 extern "C" int thd_current_status(MYSQL_THD thd);
 extern "C" enum enum_server_command thd_current_command(MYSQL_THD thd);
+extern "C" int thd_double_innodb_cardinality(MYSQL_THD thd);
 
 /**
   @class CSET_STRING