sd-bus: cleanup ssh sessions (Closes: #8076)

sd-bus: cleanup ssh sessions (Closes: #8076)

we still invoke ssh unnecessarily when there in incompatible or erreneous input
The fallow-up to finish that would make the code a bit more verbose,
as it would require repeating this bit:
```
        r = bus_connect_transport(arg_transport, arg_host, false, &bus);
        if (r < 0) {
                log_error_errno(r, "Failed to create bus connection: %m");
                goto finish;
        }

        sd_bus_set_allow_interactive_authorization(bus, arg_ask_password);
```
in every verb, after parsing.

v2: add waitpid() to avoid a zombie process, switch to SIGTERM from SIGKILL
v3: refactor, wait in bus_start_address()