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/fake_outputs.c

@@ -49,7 +49,7 @@ void fake_outputs_init(const char *output_spec) {
         } else {
             new_output = scalloc(1, sizeof(Output));
             sasprintf(&(new_output->name), "fake-%d", num_screens);
-            DLOG("Created new fake output %s (%p)\n", new_output->name, new_output);
+            DLOG("Created new fake output %s (%p)\n", output_primary_name(new_output), new_output);
             new_output->active = true;
             new_output->rect.x = x;
             new_output->rect.y = y;