basic: add RuntimeScope enum

In various tools and services we have a per-system and per-user concept.
So far we sometimes used a boolean indicating whether we are in system
mode, or a reversed boolean indicating whether we are in user mode, or
the LookupScope enum used by the lookup path logic.

Let's address that, in introduce a common enum for this, we can use all
across the board.

This is mostly just search/replace, no actual code changes.

src/hostname/hostnamectl.c

@@ -732,7 +732,7 @@ static int run(int argc, char *argv[]) {
         if (r <= 0)
                 return r;
 
-        r = bus_connect_transport(arg_transport, arg_host, false, &bus);
+        r = bus_connect_transport(arg_transport, arg_host, RUNTIME_SCOPE_SYSTEM, &bus);
         if (r < 0)
                 return bus_log_connect_error(r, arg_transport);