Get platform-dependent dir separator string.
This returns "\\" on win32, "/" on Unix, and ":" on MacOS. It may be more than one character, depending on the platform, and your code should take that into account. Note that this is only useful for setting up the search/write paths, since access into those dirs always use '/' (platform-independent notation) to separate directories. This is also handy for getting platform-independent access when using stdio calls.