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.

Reviewed by: des
Reviewed by: pstef
Differential Revision: https://reviews.freebsd.org/D38386

(cherry picked from commit 48bfd3597654490cdc43bf0f591a539d3a28b590)