* Make some minor performance improvements.

* Make the I_Errors in filesrch.c more descriptive.
* Fix up the loading of sounds.kart such that it has SOME protection against being loaded with game-modifying lumps.
* SERIOUSLY fix up the MD5/mainwads++ section in D_SRB2Main, since 1) I broke it and 2) it was already a little broken in the first place but we just didn't notice it because #ifndef DEVELOP.