dh_installtmpfiles: implement remove on removal, purge on purge

systemd-tmpfiles (since version v256) supports a --purge mode that deletes all files created by a tmpfiles.d config. Copy each tmpfiles.d in the postrm scripts, so that they can be run on package purge and remove respectively (when the respective tmpfiles.d are already deleted, hence the copy). Allow disabling either operation with --no-remove/--no-purge.