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/meson.build

@@ -71,6 +71,7 @@ basic_sources = files(
         'recurse-dir.c',
         'replace-var.c',
         'rlimit-util.c',
+        'runtime-scope.c',
         'sigbus.c',
         'signal-util.c',
         'siphash24.c',