Tighten requirements between subpackages
    
    Use %{?_isa} everywhere: automatically generated requirements do not
    specify the exact version. In principle it was possible to satisfy
    automatically added requirements with an older systemd, and our manual
    R:%{name}=%{version}-%{release} with a systemd package from a
    different architecture. So tighten the requirements to make sure all
    systemd subpackages are always upgraded in lock-step.
    
    systemd-devel needs to require systemd-compat-libs explicitly. Otherwise
    we get dangling symlinks.