Signed-off-by: Stelian Ionescu <sionescu@common-lisp.net>

- Three new modules: POSIX (code imported from CL-POSIX aka IOLIB-POSIX
  aka CFFI-UNIX), MACH, and WINDOWS.  Those last two don't have much
  though.
- Port high-level interface to CFFI and using the low-level interface
  defined in OSICAT-POSIX.
- Added new file/pathname utilities adapted from CL-FAD: WALK-DIRECTORY,
  DELETE-DIRECTORY-AND-FILES, DIRECTORY-EXISTS-P, LIST-DIRECTORY,
  FILE-EXISTS-P, DIRECTORY-PATHNAME-P, PATHNAME-AS-DIRECTORY,
  PATHNAME-AS-FILE.
- New: PATHNAME-DIRECTORY-PATHNAME and GET-MONOTONIC-TIME.