systemctl: move unit action table closer to the functions that use them

All other static tables we placed next to the functions that access
them, so let's do this for the unit action table too.
systemctl: when mangle unit names for the "isolate", suffix with ".target" rather than ".service" by default

After all, we set AllowIsolate exclusively for target units so far, and
this is more or less the only thing tht makes sense, hence also use
".target" as completion suffix by default.
update TODO
polkit: as we dropped support for the polkit bus transport, also drop its polkit policy
sd-bus: add support for new allow-interactive-authorization message header flag
bus: check allow-interactive-auhtorization flag when doing polkit