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()

src/basic/process-util.c

@@ -806,6 +806,13 @@ void sigkill_waitp(pid_t *pid) {
         sigkill_wait(*pid);
 }
 
+void sigterm_wait(pid_t pid) {
+        assert(pid > 1);
+
+        if (kill_and_sigcont(pid, SIGTERM) > 0)
+                (void) wait_for_terminate(pid, NULL);
+}
+
 int kill_and_sigcont(pid_t pid, int sig) {
         int r;