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/unit.c

@@ -4009,7 +4009,7 @@ UnitFileState unit_get_unit_file_state(Unit *u) {
 
         if (u->unit_file_state < 0 && u->fragment_path) {
                 r = unit_file_get_state(
-                                u->manager->unit_file_scope,
+                                u->manager->runtime_scope,
                                 NULL,
                                 u->id,
                                 &u->unit_file_state);
@@ -4036,7 +4036,7 @@ int unit_get_unit_file_preset(Unit *u) {
                         return (u->unit_file_preset = -EISDIR);
 
                 u->unit_file_preset = unit_file_query_preset(
-                                u->manager->unit_file_scope,
+                                u->manager->runtime_scope,
                                 NULL,
                                 bn,
                                 NULL);