1.0.12.6: Removing UNIX-NAMESTRING, part 1

* Get NATIVE-NAMESTRING to do all and only the desired things for all
  accepted non-wild NAME and TYPE components.  Add a few tests for
  these cases.

* Add new user-visible features to PARSE-NATIVE-NAMESTRING and
  NATIVE-NAMESTRING for parsing/unparsing things "as files" or "as
  directories"; these are convenient for use with SB-POSIX, and will
  be handy in a few places in SBCL's internals, too.