xargs now takes -P0, creating as many concurrent processes as possible

PR: 199976
Submitted by: Nikolai Lifanov <lifanov@mail.lifanov.com>
Reviewed by: mjg, bjk
Approved by: bapt (mentor)
MFC after: 1 month
Relnotes: yes
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D2616