New procedure PATHNAME-SIMPLIFY maps a pathname into an equivalent

New procedure PATHNAME-SIMPLIFY maps a pathname into an equivalent
simpler pathname in a host-dependent fashion.  Unix provides one
simplification: ".." directories are removed when doing so does not
change the meaning of the pathname.

Additionally, treatment of pathname hosts changed to improve
performance, and to fix problems with fasdumping of pathname objects.
Current implementation permits pathnames to be fasdumped and then
fasloaded without changing their behavior.