Fix scanning of VGs without in-PV mdas.

Set cmd->independent_metadata_areas if metadata/dirs or disk_areas in use.
- Identify and record this state.

Don't skip full scan when independent mdas are present even if memlock is set.
- Clusters and OOM aren't supported, so no problem doing the proper scans.

Avoid revalidating the label cache immediately after scanning.
- A simple optimisation.

Support scanning for a single VG in independent mdas.
- Not used by the fix but I left it in anyway as later patches might use it.

lib/commands/toolcontext.c

@@ -1138,6 +1138,7 @@ struct cmd_context *create_toolcontext(unsigned is_long_lived,
 	cmd->is_long_lived = is_long_lived;
 	cmd->handles_missing_pvs = 0;
 	cmd->handles_unknown_segments = 0;
+	cmd->independent_metadata_areas = 0;
 	cmd->hosttags = 0;
 	dm_list_init(&cmd->arg_value_groups);
 	dm_list_init(&cmd->formats);
@@ -1246,6 +1247,8 @@ static void _destroy_formats(struct cmd_context *cmd, struct dm_list *formats)
 			dlclose(lib);
 #endif
 	}
+
+	cmd->independent_metadata_areas = 0;
 }
 
 static void _destroy_segtypes(struct dm_list *segtypes)