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/systemctl/systemctl.h

@@ -51,7 +51,7 @@ extern char **arg_properties;
 extern bool arg_all;
 extern enum dependency arg_dependency;
 extern const char *_arg_job_mode;
-extern LookupScope arg_scope;
+extern RuntimeScope arg_runtime_scope;
 extern bool arg_wait;
 extern bool arg_no_block;
 extern int arg_legend;