debian: Fix the systemd unit file handling

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
debian: Centralize feature enabling/disabling

Instead of having different branches for the various features, have them
controlled centrally. The generated source package will be different,
but we'll use the same repository.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
debian/rules: Support building directly from git

When building directly from git, without exporting, configure is in
../../${SRCDIR}, update the configure invocation to find it there.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
debian: Drop hardening-includes requirement

We can build just fine with hardening flags, even when
hardening-includes is not used. So drop the build-depends and the
support code.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>