daemon(8): Add -P, -r, -u options from FreeBSD

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.