(otherwise a deadlock when ALTER writes to
binlog holding LOCK_open, it causes binlog rotation,
binlog waits for prepared transactions to commit, and commit
needs LOCK_open to check for global read lock)


mysql-test/r/flush.result:
  global read lock code now uses a dedicated mutex
mysql-test/t/flush.test:
  global read lock code now uses a dedicated mutex
sql/lock.cc:
  global read lock code now uses a dedicated mutex
sql/mysql_priv.h:
  global read lock code now uses a dedicated mutex
sql/mysqld.cc:
  global read lock code now uses a dedicated mutex
sql/sql_table.cc:
  global read lock code now uses a dedicated mutex