1.6.4:
   * Missing socket configuration (#31)
   * Add `make dist` target.
   * Use unquoted expansion of lists.
   * Search for libmilter properly.
   * Use -O2 by default.
   * Fix doc: add reference to strict_auth directive (by @aya)
   * Fix debug mode option.
   * Fix pidfile creation from the debian init script.
   * Enable dkim by default.
   * Fix issue with envfrom loading.
   * Fix inet6 sockets and address-less sockets lines.
   * Fix default configurations to the inet sockets.
   * Fix reload signal in systemd unit.