MXS-862: Make packet sequence handling automatic

Made the packet sequence number handling automatic so that it always uses
the correct one.

All functions now have documentation in them. Cleaned up code and added
comments to GSSAPI code.

server/modules/authenticator/gssapi_auth_common.c

@@ -24,6 +24,7 @@ void* gssapi_auth_alloc(void *instance)
         rval->state = GSSAPI_AUTH_INIT;
         rval->principal_name = NULL;
         rval->principal_name_len = 0;
+        rval->sequence = 0;
     }
 
     return rval;