We have 5 implementations of walk(), and 5 different docstrings.  Combined
'em.  Let's see how long it takes before they're all different again!

   Once upon a time, I put together a little function
   that tries to find the canonical filename for a given
   pathname on POSIX. I've finally gotten around to
   turning it into a proper patch with documentation.
   On non-POSIX, I made it an alias for 'abspath', as
   that's the behavior on POSIX when no symlinks are
   encountered in the path.

   Example:
   >>> os.path.realpath('/usr/bin/X11/X')
   '/usr/X11R6/bin/X'