Fix equality between #p"~" and (user-homedir-pathname) on Win32.

Don't use user-homedir-namestring on the windows version of
native-namestring, since the home directory can be specified as
C:/User/user, use (native-namestring (user-homedir-pathname)), which
will get C:\\User\\user\\ instead, making (native-namestring "~/") and
(native-namestring (user-homedir-pathname)) equal.