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.

include/maxscale/protocol/mysql.h

@@ -59,7 +59,6 @@
 #include <maxscale/router.h>
 #include <maxscale/poll.h>
 #include <maxscale/users.h>
-#include <maxscale/dbusers.h>
 #include <maxscale/version.h>
 #include <maxscale/housekeeper.h>
 #include <maxscale/utils.h>
@@ -93,6 +92,7 @@ MXS_BEGIN_DECLS
 #define GW_MYSQL_SCRAMBLE_SIZE 20
 #define GW_SCRAMBLE_LENGTH_323 8
 
+/** Name of the default server side authentication plugin */
 #define DEFAULT_MYSQL_AUTH_PLUGIN "mysql_native_password"
 
 /** All authentication responses are at least this many bytes long */
@@ -105,7 +105,12 @@ MXS_BEGIN_DECLS
 # define MYSQL_SCRAMBLE_LEN GW_MYSQL_SCRAMBLE_SIZE
 #endif
 
-#define MYSQL_HOSTNAME_MAXLEN  60
+/* Max length of fields in the mysql.user table */
+#define MYSQL_USER_MAXLEN     128
+#define MYSQL_PASSWORD_LEN    41
+#define MYSQL_HOST_MAXLEN     60
+#define MYSQL_DATABASE_MAXLEN 128
+#define MYSQL_TABLE_MAXLEN    64
 
 #define GW_NOINTR_CALL(A)       do { errno = 0; A; } while (errno == EINTR)
 #define SMALL_CHUNK 1024