sd-bus: allow connecting to the pseudo-container ".host"

machined exposes the pseudo-container ".host" as a reference to the host
system, and this means "machinectl login .host" and "machinectl shell
.host" get your a login/shell on the host. systemd-run currently doesn't
allow that. Let's fix that, and make sd-bus understand ".host" as an
alias for connecting to the host system.

src/basic/util.c

@@ -230,6 +230,11 @@ int container_get_leader(const char *machine, pid_t *pid) {
         assert(machine);
         assert(pid);
 
+        if (streq(machine, ".host")) {
+                *pid = 1;
+                return 0;
+        }
+
         if (!machine_name_is_valid(machine))
                 return -EINVAL;