OS/2 EMX port changes (Modules part of patch #450267):

  Modules/
    posixmodule.c

- use SEP,ALTSEP #defines instead of hard coded path separator chars
- use EMX specific variants of chdir2(),getcwd() that support drive letters
- OS/2+EMX spawnv(),spawnve() support
- EMX specific popen[234]() derived from Win32 popen[234]() code