First cut at making search-list a pathname host.

This attempts to make a search-list pathname fill the host slot of a
pathname with a search-list object instead of the current scheme which
uses a unix host for the host and puts the search-list as the first
part of the directory slot.

 code/pathname.lisp::
  * Make SEARCH-LIST as subtype of HOST, defining appropriate parsers
    and unparsers.
 code/filesys.lisp:
  * Update PARSE-UNIX-NAMESTRING (which also handles search-lists) to
    return the search-list object as the host instead of putting in
    the directory part.