Manage process-related IDs with bitmaps

Currently unique pid allocation on fork often requires a full walk of
process, group, session lists to make sure it is not used by anything.
This has a side effect of requiring proctree to be held along with allproc,
which adds more contention in poudriere -j 128.

The patch below implements trivial bitmaps which gets rid of the problem.
Dedicated lock is introduced to manage IDs.

While here a bug was discovered: all processes would inherit reap id from
the first process spawned by init. This had a side effect of keeping the
ID used and when allocation rolls over to the beginning it keeps being
skipped.

The patch is loosely based on initial work by mjoras@.

Reviewed by: kib
Sponsored by: The FreeBSD Foundation