Rewrite COM_FileExtention()

COM_FileExtension() was parsing strings from beginning to end, bailing
out as soon as '.' was found and treating everything thereafter as the
file extension. That behavior caused problem with relatives pathes like
models/monsters/tank/../ctank/skin.pcx. The new implementation uses
strrchr() to determine the last '.'.

This fixes issue #48. The bug was introduced in e07294b which replaced
hand rolled code with COM_FileExtention().