Recognize NERVE.WAD by its header+directory SHA-1 hash and defer loading of ENRVMETA to post-W_InitMultipleFiles so that it plays correctly even when added with -file rather than using the managed mission pack system.

git-svn-id: http://mancubus.net/svn/hosted/eternity/trunk@2609 86ba831e-150c-0410-ab35-e06eaae15ed9