MS-Windows followup to stpcpy changes.

 src/w32proc.c (sys_spawnve, get_lcid_callback): Use strcpy instead
 of strcat.
 src/w32menu.c (add_menu_item): Use stpcpy instead of strcat.
 src/w32.c (sys_readdir, stat_worker, symlink): Use strcpy instead of
 strcat.

 nt/gnulib.mk (stpcpy, string): Sync with the latest change in
 lib/gnulib.mk.