Since libsystem-shared is an internal implementation detail, do not
generate a shlibs file for it. This means dh_shlibdeps needs to be told
explicitly where it can find libsystemd-shared.
Mark this new package as Multi-Arch: same.
Closes: #990547
Query the upstream Makefile instead of making assumptions about how we craft
the Debian version of daily and release builds.
Rename DEB_VERSION_UPSTREAM to SHARED_LIB_VERSION to clarify that this is not a
standard dpkg variable.