a safer way of initing the mutexes in MYSQL_LOG.
is_open() is now always thread-safe.
See each file for details.


sql/handler.cc:
  is_open() with locks
sql/item_func.cc:
  is_open() with locks
sql/log.cc:
  No more 'inited'.
  We now always use is_open() in a thread-safe manner.
  This simplifies some functions (no more need to test is_open() twice).
sql/log_event.cc:
  is_open() with locks
sql/mysqld.cc:
  Init mutexes for the global MYSQL_LOG objects.
  We care about no_rotate, because we can't do it in open() anymore (because
  we don't have 'inited' anymore).
sql/repl_failsafe.cc:
  is_open() with locks
sql/slave.cc:
  init pthread objects (mutexes, conds) in the constructor of st_relay_log_info.
  Some better locking in rotate_relay_log().
sql/sql_base.cc:
  is_open() with locks
sql/sql_class.h:
  Before, we inited LOCK_log in MYSQL_LOG::open(), so in other places of the code
  when we were never 100% sure that it had been inited. For example, if the server
  was running without --log-bin, ::open() was not called so the mutex was not
  inited. We could detect it with !inited, but not safely as 'inited' was not
  protected by any mutex.
  So now:
  we *always* init the LOCK_log mutex, even if the log is not used. We can't init
  the mutex in MYSQL_LOG's constructor, because for global objects like
  mysql_bin_log, mysql_log etc, the constructor is called before MY_INIT(), but
  safe_mutex depends on MY_INIT(). So we have a new function MYSQL_LOG::init_pthread_objects
  which we call in main(), after MY_INIT().
  For the relay log, we call this function in the constructor of
  st_relay_log_info, which is called before any function tries to
  use the relay log (the relay log is always invoked as rli.relay_log).
  So now we should be safe in all cases and we don't need 'inited'.
sql/sql_db.cc:
  is_open() with locks
sql/sql_delete.cc:
  is_open() with locks
sql/sql_insert.cc:
  is_open() with locks
sql/sql_load.cc:
  is_open() with locks
sql/sql_parse.cc:
  is_open() with locks
sql/sql_rename.cc:
  is_open() with locks
sql/sql_repl.cc:
  is_open() with locks
sql/sql_table.cc:
  is_open() with locks
sql/sql_update.cc:
  is_open() with locks