gpg,w32: Fix gnupg_remove.

* common/sysutils.c (map_w32_to_errno): New.
(gnupg_w32_set_errno): New.
(gnupg_remove) [w32]: Set ERRNO
--

To support Unicode gnupg_remove was changed to use DeleteFileW and not
properly tested because the code was alreadt used in Windows CE.
However, ERRNO was not set and thus Dirmngr failed due to

 if (!gnupg_remove (fname))
   log_info (_("removed stale te[...] file '%s'\n"), fname);
 else if (errno != ENOENT)
   {
     err = gpg_error_from_syserror ();
     log_error (_("problem remov[...] file '%s': %s\n"),
                fname, gpg_strerror (err));
     goto leave;
   }

GnuPG-bug-id: 5230
(cherry picked from commit b6967d3191)

common/dotlock.c

@@ -437,6 +437,8 @@ static int never_lock;
 
 
 #ifdef HAVE_DOSISH_SYSTEM
+/* FIXME: For use in GnuPG this can be replaced by
+ *        gnupg_w32_set_errno.  */
 static int
 map_w32_to_errno (DWORD w32_err)
 {