MFC: r285552
Use strtonum(3) instead of atoi(3)

MFC: r286289
Introduce -P0, creating as many concurrent processes as possible

Relnotes: yes

MFC: r286461
Fix regression in xargs -Px, add more regression tests

MFC: r287004
MFC: r287005
Fix the racy xargs -P0 -n2 test added in r286289

PR: 199976
Submitted by: Nikolai Lifanov <lifanov@mail.lifanov.com>