Move pickupfx processing code to e_inventory in preparation for refactoring.

git-svn-id: http://mancubus.net/svn/hosted/eternity/trunk@2578 86ba831e-150c-0410-ab35-e06eaae15ed9
Call strerror if fopen fails in D_CheckIWAD.

git-svn-id: http://mancubus.net/svn/hosted/eternity/trunk@2579 86ba831e-150c-0410-ab35-e06eaae15ed9
Paths which fail stat will no longer be available for selection from the IWAD picker.

git-svn-id: http://mancubus.net/svn/hosted/eternity/trunk@2580 86ba831e-150c-0410-ab35-e06eaae15ed9