mysql_client.c:gw_client_close didn't close client session in cases where session->state == SESSION_STATE_STOPPING. That is a bug and lead to situation where session wasn't closed at all.
	Also changed 'authorization failed' to 'access denied'
mysql_common.c: fixed memory leak in gw_receive_backend_auth, and replaced error code '2800' with '28000'.
readconnroute.c:handleError didn't set *succp pointer so uninitialized value was used in caller's context.
makefile.inc: added -lm to linker flags
mysql_backend.c: added a few comments
Merge branch 'develop' of https://github.com/mariadb-corporation/MaxScale into develop