Extend and implement the grammar that allows to FLUSH WITH READ LOCK
a list of tables, rather than all of them.

Incompatible grammar change:
Previously one could perform FLUSH TABLES, HOSTS, PRIVILEGES in a single
statement.
After this change, FLUSH TABLES must always be alone on the list.
Judging by the test suite, however, the old extended syntax
was never or very rarely used.

The new statement requires RELOAD ACL global privilege and
LOCK_TABLES_ACL | SELECT_ACL on individual tables.
In other words, it's an atomic combination of LOCK TALBES <list> READ
and FLUSH TABLES <list>, and requires respective privileges.

For additional information about the semantics, please
see WL#5000 and the comment for flush_tables_with_read_lock()
function in sql_parse.cc