Provide an alternative to exiting immediately upon save_cwd or

Provide an alternative to exiting immediately upon save_cwd or
restore_cwd failure.  Now, an application can arrange e.g.,
to perform a longjump in that case.
* openat.c: Include dirname.h.
Use IS_ABSOLUTE_FILE_NAME rather than testing for leading slash.
(rpl_openat, fdopendir, fstatat): Call openat_save_die
and openat_restore_die rather than calling error directly.
Don't include "error.h" or "exitfail.h"; they're no longer needed.

* openat-die.c (openat_save_die, openat_restore_die): New file.
* openat.h (openat_save_die, openat_restore_die): Declare and define.