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/xinerama.c

@@ -56,7 +56,7 @@ static void query_screens(xcb_connection_t *conn) {
         } else {
             s = scalloc(1, sizeof(Output));
             sasprintf(&(s->name), "xinerama-%d", num_screens);
-            DLOG("Created new Xinerama screen %s (%p)\n", s->name, s);
+            DLOG("Created new Xinerama screen %s (%p)\n", output_primary_name(s), s);
             s->active = true;
             s->rect.x = screen_info[screen].x_org;
             s->rect.y = screen_info[screen].y_org;