user-homedir-pathname not amenable to merging.

a reminder for gary from our chat on #lisp --

(user-homedir-pathname) and #P"/Users/bryan/" differ by
%PATHNAME-NAME and %PATHNAME-TYPE being set to :UNSPECIFIED
in the former, NIL in the latter.

this seems to break (merge-pathnames ...).

see transcript below.

Welcome to OpenMCL Version (Beta: Darwin) 0.13.4!
? (user-homedir-pathname)
#1P"/Users/bryan/"
? (merge-pathnames (user-homedir-pathname) #P"foo")
#1P"/Users/bryan/"
? (merge-pathnames #P"/Users/bryan/" #P"foo")
#P"/Users/bryan/foo"
? (describe (user-homedir-pathname))
#1P"/Users/bryan/"
Type: PATHNAME
Class: #<BUILT-IN-CLASS PATHNAME>
TYPE: PATHNAME
%PATHNAME-DIRECTORY: (:ABSOLUTE "Users" "bryan")
%PATHNAME-NAME: :UNSPECIFIC
%PATHNAME-TYPE: :UNSPECIFIC
? (describe #P"/Users/bryan/")
#P"/Users/bryan/"
Type: PATHNAME
Class: #<BUILT-IN-CLASS PATHNAME>
TYPE: PATHNAME
%PATHNAME-DIRECTORY: (:ABSOLUTE "Users" "bryan")
%PATHNAME-NAME: NIL
%PATHNAME-TYPE: NIL
? (quit)