idStr::StripFileExtension() (and SetFileExtension() which uses it) and
others didn't work correctly if there was a dot in a directory name,
because they just searched from last to first char for '.', so if the
current filename didn't have an extension to cut off, they'd just cut
off at any other '.' they found.
So D:\dev\doom3.data\base\maps\bla could turn into D:\dev\doom3
(or, for SetFileExtension(), D:\dev\doom3.map)
While at it, I made most of the idStr code that explicitly checked for
'\\' and '/' (and maybe ':' for AROS) use a little
"bool isDirSeparator(int c)" function so we don't have the #ifdefs
for different platforms all over the place.
431de18a60