Read configuration variables from sysconfig

Both directories /etc/sysconfig and /etc/default are searched for by
systemd and if syslog-ng exists it will be used for extracting env
variables. This should cover debian and redhat based systems and is
convenient to migrate from sysvinit to systemd

contrib/systemd/syslog-ng.service

@@ -6,6 +6,8 @@ Documentation=man:syslog-ng(8)
 Type=notify
 ExecStart=/usr/sbin/syslog-ng -F
 ExecReload=/bin/kill -HUP $MAINPID
+EnvironmentFile=-/etc/default/syslog-ng
+EnvironmentFile=-/etc/sysconfig/syslog-ng
 StandardOutput=journal
 StandardError=journal
 Restart=on-failure