Fix training restrictions in skill menus and messages for Gnolls (eb)

If Gnolls changed their inventory, spells, or abilities in a way that
modify the usual training restrictions, their skill menu would show the
skill without the '+' indicated it was being trained and they would get
a message to that effect, although they would actually continue training
the skill normally. This commit prevents those messages and skill menu
changes.

It may need to be done in a better way elsewhere in the skill code, but
this fix will work for now.

Author: gammafunk

crawl-ref/source/skills.cc

@@ -506,6 +506,10 @@ static void _check_start_train()
 
 static void _check_stop_train()
 {
+    // Gnolls can't stop training skills.
+    if (you.species == SP_GNOLL)
+        return;
+
     _check_inventory_skills();
     _check_spell_skills();
     _check_abil_skills();