debian: Switch to dh auto sequencer

debian: Switch to dh auto sequencer

We drop much code from debian/rules.  In particular, we now use the
upstream `make install` targets.  (As a side effect, we discovered
some bugs ion the upstream Makefile which have just been fixed in the
commits leading up to this one.)

Consequences for the generated binaries:

 * Upstream README is shipped as /usr/share/doc/userv/README,
   not README.upstream.  This seems conventional in Debian nowadays.
 * Control file md5sums are provided.
 * There is a new changelog symlink for changelog.Debian.gz.
 * We use the proper Debian debug information scheme.
 * We use different compiler flags, which will hopefully produce
   better hardening and better reproducibility.

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