WAD detection (I_FindFile) should not be case sensitive

[schmurtzm]

Description :
Many original Doom wad files were in uppercase but prboom detects only the .wad in lower case. For example if you have DOOM.WAD and try to start SIGIL then the core will not detect doom.wad and SIGIL will not run. You have to rename DOOM.WAD in doom.wad or doomu.wad. I suppose that it is only a problem for linux system and not Windows but it still very annoying to have to change the original case to make it work.

Required solution :
Be able to detect the main wad files even in upper case.

Step to reproduce :
install SIGIL next to an DOOM.WAD file in uppercase.

Platform :
any linux device