daemon: Allow logging daemon stdout/stderr to file or syslog.

There are various new options, documented in the man page, to send the
daemon's standard output and/or standard error to a file or to syslog.

Submitted by: ank at iki.fi
Reviewed by: wblock (man page only)
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D7993