nproc: port some Linux code path to freebsdlike

[oech3]

No description provided.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/basenc/bounded-memory is now being skipped but was previously passing.
Congrats! The gnu test tests/cp/sparse-2 is now passing!
Congrats! The gnu test tests/tail/pipe-f is now passing!
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.

[oech3]

Maybe, not useful