Remove timestamp from default FreeBSD and OpenBSD log

[EugeneOne1]

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.