load-fragment: allow to specify RestrictNamespaces= multiple times

If multiple RestrictNamespaces= settings are set, then merge the settings.
This also drops supporting "~yes" and "~no".

src/core/execute.c

@@ -3562,7 +3562,8 @@ void exec_context_init(ExecContext *c) {
         for (i = 0; i < _EXEC_DIRECTORY_TYPE_MAX; i++)
                 c->directories[i].mode = 0755;
         c->capability_bounding_set = CAP_ALL;
-        c->restrict_namespaces = NAMESPACE_FLAGS_ALL;
+        assert_cc(NAMESPACE_FLAGS_INITIAL != NAMESPACE_FLAGS_ALL);
+        c->restrict_namespaces = NAMESPACE_FLAGS_INITIAL;
         c->log_level_max = -1;
 }