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/core/execute.c

@@ -4448,7 +4448,7 @@ static int exec_child(
          * invocations themselves. Also note that while we'll only invoke NSS modules involved in user management they
          * might internally call into other NSS modules that are involved in hostname resolution, we never know. */
         if (setenv("SYSTEMD_ACTIVATION_UNIT", unit->id, true) != 0 ||
-            setenv("SYSTEMD_ACTIVATION_SCOPE", MANAGER_IS_SYSTEM(unit->manager) ? "system" : "user", true) != 0) {
+            setenv("SYSTEMD_ACTIVATION_SCOPE", runtime_scope_to_string(unit->manager->runtime_scope), true) != 0) {
                 *exit_status = EXIT_MEMORY;
                 return log_unit_error_errno(unit, errno, "Failed to update environment: %m");
         }