Update Debian package

Update Debian package

- Update debhelper compatibility to version 9.
- Update build-dependencies.
- Change group of ngircd.conf to "irc" in postinst script (this makes
  starting ngIRCd as user "irc" easier, for example when using systemd).
- Don't create /var/run/ircd during installation: this is handled by the
  SysV init script and the systemd service already.