daemon(8): Add -P, -r, -u options from FreeBSD
This is a full sync with FreeBSD. The DragonFly daemon hasn't been
significantly updated in years. The sync brings in:
option -P supervisor pidfile
This writes the pid of the daemon process in the specified file.
It gets removed automatically similarly to the child pidfile
option -u user
It executes the command using the credentials the the specified
user, and it requires the superuser to invoke it.
option -r restart
This acts like a watchdog and restarts a program that terminates
The update requires an update to libutil in order to use the same
pidfile handling that FreeBSD uses.