Move dbusers.c out of the core

The dbusers.c was a MySQL protocol specific file which was used directly
by some of the modules.

Added a new return value for the loadusers authenticator entry point which
allows fatal failures to occur when users are loaded. Currently this is
only taken into notice when the service is first started. If a listener
later returns a fatal error, it is only logged but the service stays in
operation.

Moved the MySQLAuth authenticator sources and the tests that relate to
this module into a subdirectory in the authenticator
directory. Eventually, all authenticators could have a subdirectory of
their own.

server/core/mysql_binlog.c

@@ -22,7 +22,6 @@
 #include <string.h>
 #include <maxscale/debug.h>
 #include <maxscale/users.h>
-#include <maxscale/dbusers.h>
 #include <strings.h>
 
 /**