Add MXS_ALERT

As errors and warnings are throttled, there is a need for being able
to log severe errors without ever having them throttled (e.g. when
logging the stack in conjunction with a crash).

MXS_ALERT should only be used in a context where the process is known
to be going down, either via crash or explicit exit.

server/core/log_manager.cc

@@ -548,7 +548,7 @@ static bool logmanager_init_nomutex(const char* ident,
     /**
      * Set global variable
      */
-    mxs_log_enabled_priorities = MXS_LOG_ERR | MXS_LOG_NOTICE | MXS_LOG_WARNING;
+    mxs_log_enabled_priorities = (1 << LOG_ERR) | (1 << LOG_NOTICE) | (1 << LOG_WARNING);
 
     /**
      * Initialize filewriter data and open the log file