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-start-unit.c

@@ -168,8 +168,8 @@ fail:
                                            BUS_ERROR_UNIT_MASKED,
                                            BUS_ERROR_JOB_TYPE_NOT_APPLICABLE))
                 log_error("See %s logs and 'systemctl%s status%s %s' for details.",
-                          arg_scope == LOOKUP_SCOPE_SYSTEM ? "system" : "user",
-                          arg_scope == LOOKUP_SCOPE_SYSTEM ? "" : " --user",
+                          runtime_scope_to_string(arg_runtime_scope),
+                          arg_runtime_scope == RUNTIME_SCOPE_SYSTEM ? "" : " --user",
                           name[0] == '-' ? " --" : "",
                           name);
 
@@ -242,7 +242,7 @@ static const char** make_extra_args(const char *extra_args[static 4]) {
 
         assert(extra_args);
 
-        if (arg_scope != LOOKUP_SCOPE_SYSTEM)
+        if (arg_runtime_scope != RUNTIME_SCOPE_SYSTEM)
                 extra_args[n++] = "--user";
 
         if (arg_transport == BUS_TRANSPORT_REMOTE) {