Create separate standalone packages for tmpfiles and sysusers
    
    These packages include binaries that link to a static version of
    libsystemd-shared, so they don't depend on the systemd-libs package at
    runtime.
    
    These packages are intended to expose systemd-tmpfiles and systemd-sysusers
    to non-systemd systems, such as container images.
    
    Note that static linking only pulls in the small subset of functions from
    libsystemd-shared that are actually used by the binaries, so the total size of
    a statically linked binary is much smaller than the sum of the shared binary
    with the shared library. The resulting binaries on an x86_64 build have 272KB
    (tmpfiles) and 180KB (sysusers).
    
    This commit relies on the -Dstandalone-binaries=true build configuration that
    was pushed upstream in PR 16061 and released in systemd v246.