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/machine/machine-dbus.c

@@ -502,7 +502,7 @@ static int container_bus_new(Machine *m, sd_bus_error *error, sd_bus **ret) {
                 bus->address = address;
                 bus->bus_client = true;
                 bus->trusted = false;
-                bus->is_system = true;
+                bus->runtime_scope = RUNTIME_SCOPE_SYSTEM;
 
                 r = sd_bus_start(bus);
                 if (r == -ENOENT)

src/machine/machinectl.c

@@ -2842,7 +2842,7 @@ 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);