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/basic/runtime-scope.c

@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "runtime-scope.h"
+#include "string-table.h"
+
+static const char* const runtime_scope_table[_RUNTIME_SCOPE_MAX] = {
+        [RUNTIME_SCOPE_SYSTEM] = "system",
+        [RUNTIME_SCOPE_USER]   = "user",
+        [RUNTIME_SCOPE_GLOBAL] = "global",
+};
+
+DEFINE_STRING_TABLE_LOOKUP(runtime_scope, RuntimeScope);