Introduce output_primary_name function

Currently simply returns output->name, but this will make it easier to
change how output names are stored in the following commits.

Also replace reading output->name with invocations of
output_primary_name. Code which writes output->name is unchanged. Done
using a mostly mechanical replacement of output->name to
output_primary_name(output).

src/manage.c

@@ -219,7 +219,7 @@ void manage_window(xcb_window_t window, xcb_get_window_attributes_cookie_t cooki
         LOG("This window is of type dock\n");
         Output *output = get_output_containing(geom->x, geom->y);
         if (output != NULL) {
-            DLOG("Starting search at output %s\n", output->name);
+            DLOG("Starting search at output %s\n", output_primary_name(output));
             search_at = output->con;
         }