fix regression in xargs -Px and add regression tests

[allanjude]

Summary

jbeich reported that xargs -Px doesn't work anymore because
of a botched rebase. This is true, but easy to fix. See attached patch.
I also added test cases to xargs to test -P functionality in the future.

There is Bugzilla 202152 about this.

Test Plan

kyua -k /usr/tests usr.bin/xargs

[jbeich]

• Code looks OK but see inline nit.
• Tests look OK for rS286289 scope as it didn't touch how processes are spawned.

Note, I'm not a src/ committer to have much weight.

usr.bin/xargs/xargs.c

174 ↗

(On Diff #7754)

Unreachable as errstr conditional above already checks valid range:

$ printf 'a\nb\nc\n' | xargs -P -1 echo
xargs: -P -1: too small

[lifanov_mail.lifanov.com]

You are right, maxprocs < 0 is unreachable

[lifanov_mail.lifanov.com]

You are right, maxprocs < 0 is unreachable. I updated the diff.

[allanjude]

In the process of being switched from strtoul() to strtonum(), a check against endptr was not removed, causing xargs to segfault

This patch fixes that, and adds a set of regression tests against the -P flag, that were previously missing.

I would like to commit this, to fix the previous commit.