syslogd: Watch for dead pipe processes

For each new pipe process, add its process descriptor into the kqueue
with the EVFILT_PROCDESC filter and NOTE_EXIT event. When the pipe
process exits, the main kqueue loop will catch this, logging exit errors
and cleaning up the pipe process' filed node.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D41477