* Use syslog to manage caddy std{out,err} on FreeBSD

There is no good way to rotate the logfile created by the previous
FreeBSD rc.d script (it's the result of redirecting std{out,err} and
is held open by the shell).

This solves the problem by sending caddy's std{out,err} stream to
syslog, using the daemon command's builtin functionality.

It replaces the old `caddy_logfile` rc.conf variable with
`caddy_syslog_facility` (which defaults to 'local7') and
`caddy_syslog_level` (which defaults to 'notice').

By default, these messages will end up in /var/log/messages but can
be redirected as documented in the script's comments.

* Add info about rotating log with newsyslog

If you create a caddy specific logfile in `/var/log`, you should
rotate it.

This adds a bit of info to the dist/init/freebsd/README.md about
rotating that log file with newsyslog.