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;
}

(cherry picked from commit b6967d31912912ad3c0a2ff6bf6eb9822a194562)

• GnuPG-bug-id: T5230