F/add systemd source

I talked with Bazsi about this IRL, and we decided to move this functionality into a new function in systemd-syslog source, called `create_and_set_unix_dgram_or_systemd_source`.

In case of `systemd-source`, users can set the configuration options, but they will be ignored and a warning message will inform them about that.

Commits:
move out the logic: 824a634744b9b8833095dcb472c1e692435d5a07
ignore configuration: 9c270fa4e38431e202a6fe9262648df9f14eba34