Rewrite part of the packaging to use `debhelper` classic

Rewrite part of the packaging to use `debhelper` classic

This rewrites `debian/rules` to use `debhelper` more. With this,

 1) The package now builds without requiring root.
 2) The package now produces dbgsym packages
 3) The package now includes the md5sums control member.
 4) The package now uses dh_clean to properly clean up after the
    debhelper commands.

There are no loess of functionality or features in any of the debs.
The only delta to the original debs is that the original debs uses
"non-standard" mode for symlinks (like we saw with authbind). However,
that mode is ignored on unpack, so the delta is irrelevant.

The `debian/chiark-backup/conffiles` file was made obsolete by using
`dh_installdeb`, which generates that content automatically
(admittedly, with a slightly different order, but the order is
semantically irrelevant).

One downside with this approach is that the `binary-one` target has
to restore the setuid mode of `really`, which is a bit awkward in the
current design. Long term is to remove the `binary-one` target, but that
will when the `debian/tmp/$PKG` is rewritten to be `debian/$PKG`, so the
debhelper tools work better out of the box with those directories.

Closes: #1089303
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>