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/ipc.c

@@ -829,7 +829,7 @@ IPC_HANDLER(get_outputs) {
         y(map_open);
 
         ystr("name");
-        ystr(output->name);
+        ystr(output_primary_name(output));
 
         ystr("active");
         y(bool, output->active);