stdlib-safer: wrap all mkstemp variants

stdlib-safer: wrap all mkstemp variants

* modules/mkostemp (configure.ac): Set witness.
* modules/mkostemps (configure.ac): Likewise.
* modules/mkstemps (configure.ac): Likewise.
* lib/stdlib-safer.h (mkostemp_safer, mkostemps_safer)
(mkstemps_safer): Wrap more functions.
* lib/stdlib--.h (mkostemp, mkostemps, mkstemps): Default the
wrapping.
* lib/mkstemp-safer.c (mkostemp_safer, mkostemps_safer)
(mkstemps_safer): Implement the wrappers.

Signed-off-by: Eric Blake <ebb9@byu.net>