Merge branch 'systemd'

* systemd:
  ngircd.sock: explicitely bind to IPv4 and IPv6 addresses
  Show address and port of sockets passed-in by systemd(8)
  Check type of sockets passed-in by systemd(8)
  Adjust severity levels of some log messages
  New configuration option "IdleTimeout": exit daemon when idle
  Implement support for systemd(8) "socket activation"
  contrib/README: add more files