Only use the gi_menuhack_retail flag check where contextually appropriate.  It's technically not needed at all, but particularly screwed up some vanilla compat when checking the flag vs. checking the gamemode variable.  Also fix a slight error on the server for when to fall over to the next episode (retail vs. registered).

git-svn-id: http://odamex.net/svn/root/trunk@3734 b0bcef46-2027-0410-bd56-ab726e9a1d04