Clean config.h some more

Moved some typedefs to router.h and server.h, changed a few
constants to these enums. Renamed some types in config.h to
remove "Gateway".

There are still some functions in the public header which are
only used in core, but they seem to fit the theme of public functions
so were not moved.

include/maxscale/server.h

@@ -99,6 +99,12 @@ typedef struct server
 #endif
 } SERVER;
 
+enum
+{
+    MAX_RLAG_NOT_AVAILABLE = -1,
+    MAX_RLAG_UNDEFINED = -2
+};
+
 /**
  * Status bits in the server->status member.
  *