Rename rc.conf variables syslog_ng_config and syslog_ng_pid to
syslog_ng_flags and syslog_ng_pidfile respecively, making them
consistent with the rest of the ports tree.

PR: 249984
Submitted by: Boris Korzun <drtr0jan@yandex.ru>
Reviewed by: Peter Czanik (syslog-ng upstream)