Revert "Fixed|Unix|FS1: Switch to correct path separators"

This commit (c35524ea5cc7d02500a8881e86f7b5718893c394) results in
a fatal error on Windows during startup. Now reverted.

We'll have to look elsewhere for a solution to the Unix-only bug
where external music files could not be found in the Classic music
packs.
Merge branch 'master' of github.com:skyjake/Doomsday-Engine