systemd-source: Add systemd source driver.

Signed-off-by: Tibor Benke <ihrwein@gmail.com>
system-source: Change unix-dgram("/dev/log") source to systemd() source
    on Linux, if systemd is currently used.

Signed-off-by: Tibor Benke <ihrwein@gmail.com>
afsocket: Auto change from unix-dgram("/dev/log") to systemd() source,
    when systemd is used.

Signed-off-by: Tibor Benke <ihrwein@gmail.com>