* Migrate to a common init script for all server versions, to avoid

  providing the "postgresql" service in multiple packages (which causes
  insserv to complain bitterly):
  - debian/init.d-functions: Add function get_versions() which returns all
    installed versions, but filters out the ones which have their own init
    script. With that, we can run newer common versions with older server
    packages.
  - debian/maintscripts-functions: Add function stop_version() (to be called
    from "prerm upgrade"), and start cluster in configure_version() if there
    is no version specific init script.
  - Add debian/postgresql-common.postgresql.init: Common init script for all
    installed server versions.
  - debian/rules: Install common init script.
  - (Closes: #XXXXXX)