syslogd: Use process descriptors

Prepare for program Capsicumization by storing process descriptors
instead of pids. Signal delivery is not permitted in capability mode,
so we can use pdkill(2) to terminate child processes.

Reviewed by: markj
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41369