libbsd: add strtonum(), nitems(), timespec operation functions from FreeBSD

Change-Id: I56b677b382d17a27d243b615de19bec10f3a2810
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7426
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: waddlesplash <waddlesplash@gmail.com>