systemd-syslog: use systemd_syslog as keyword and print an error message, when users want to use systemd-syslog source, but syslog-ng wasn't compiled with --enable-systemd flag.
systemd-syslog: add copyright header
systemd-syslog: use SD_LISTEN_FDS_START instead of systemd_syslog_sd_get_fd() wrapper function.
systemd-syslog: change system_sysblock_add_linux() to return void

Signed-off-by: Tibor Benke <ihrwein@gmail.com>
afsocket: move logic of auto changing from unix-dgram('/dev/log') to systemd-syslog source
systemd-syslog: ignore socket options set from the configuration
service-management: detect systemd by checking the existence of /run/systemd/system