Bug#11733 (COMMITs should not happen if read-only is set)
Bug#22009 (Can write to a read-only server under some circumstances)

See the work log for details

The change consist of
a) acquiring the global read lock in SET GLOBAL READONLY
b) honoring opt_readonly in ha_commit_trans(),
c) honoring opt_readonly in mysql_lock_tables().

a) takes care of the server stability,
b) makes the transactional tables safe (Bug 11733)
c) makes the non transactional tables safe (Bug 22009)


mysql-test/r/read_only.result:
  WL#3602 (SET GLOBAL READONLY)
mysql-test/t/read_only.test:
  WL#3602 (SET GLOBAL READONLY)
sql/handler.cc:
  WL#3602 (SET GLOBAL READONLY)
sql/lock.cc:
  WL#3602 (SET GLOBAL READONLY)
sql/set_var.cc:
  WL#3602 (SET GLOBAL READONLY)
sql/set_var.h:
  WL#3602 (SET GLOBAL READONLY)
mysql-test/r/read_only_innodb.result:
  WL#3602 (SET GLOBAL READONLY)
mysql-test/t/read_only_innodb.test:
  WL#3602 (SET GLOBAL READONLY)