systemd

When building in upstream mode, the release doesn't really have any
meaning so let's stop passing it as part of the version-tag and
shared-library-tag arguments.

This also makes it possible to make the release a timestamp so that
each package built from upstream is guaranteed to be newer. If we
pass the release to meson via version-tag and shared-library-tag and
the release changes every build, we end up having constant rebuilds
of various targets in meson that depend on the version.