getconf: Add NPROCESSORS_{CONF,ONLN} [BZ #31661]

These are required by the upcoming POSIX standard and are available on
other platforms.

Link: https://austingroupbugs.net/view.php?id=339


Signed-off-by: Mohamed Akram <mohd.akram@outlook.com>
Reviewed-by: Arjun Shankar <arjun@redhat.com>