Remove timestamp from default FreeBSD and OpenBSD log

In #4213 we've introduced the support for the default daemon logging mechanism for FreeBSD and OpenBSD. AdGuardHome now uses the default `logger(1)` on both systems writing its own log along with other daemons of the OS.

AdGuardHome also always prepends the timestamp to its log entries as well as `logger(1)` does. It causes some redundant output which we should get rid of. Note that this should only be done if the OS is **FreeBSD** or **OpenBSD** and the default logger is used.

Consider also adding a flag for running as service.