Resolve pathnames with :back.

Remove as much :back as possible at MAKE-PATHNAME time.

Handle any left :back in UNPARSE-NATIVE-UNIX/WIN32-NAMESTRING the same as
:up.
improve generic function encapsulation

Perform the encapsulation in an :AROUND method specialized on
STANDARD-GENERIC-FUNCTION, so that subclasses transparently inherit
encapsulation handling while still being able to override the standard
COMPUTE-DISCRIMINATING-FUNCTION behaviour.  (This is method structure
that AMOP allows implementations, as the :AROUND method is an
extending one -- it calls CALL-NEXT-METHOD)
define sane escaping in win32 pathname namestrings

We can't use \\ to escape, because it's the directory separator; that
way lies madness.  There's some evidence from random web searching that
win32 users might not be too confused with ^ as an escape character, so
try that.