kinfo_proc: move job-control related data collection into a new helper.

This improves code structure and allows to put the lock asserts right
into place where the locks are needed.

Also move zeroing of the kinfo_proc structure from fill_kinfo_proc_only()
to fill_kinfo_proc(), this looks more symmetrical.

Reviewed by: jilles
Tested by: pho
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27871