Add nproc(1)

This program prints the number of CPU threads it can run on, while
respecting cpusets (or not, depending on switches).

It aims to be compatible with nproc as found in GNU coreutils.

Approved by: re (cperciva)
Reviewed by: des
Reviewed by: pstef
Differential Revision: https://reviews.freebsd.org/D38386

(cherry picked from commit 48bfd3597654490cdc43bf0f591a539d3a28b590)
(cherry picked from commit 31f9db72a35bb20ad01d792eaaa1d6048252ab38)