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/sysutils.h

@@ -106,6 +106,7 @@ int gnupg_inotify_has_name (int fd, const char *name);
 
 
 #ifdef HAVE_W32_SYSTEM
+void gnupg_w32_set_errno (int ec);
 void *w32_get_user_sid (void);
 
 #include "../common/w32help.h"