procctl: add state flags to PROC_REAP_GETPIDS reports

For a process supervisor using the reaper API to track process subtrees,
it is very useful to know the state of the processes on the list.

Sponsored by: https://www.patreon.com/valpackett
Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39585