For pcmk__designated_controller(), it seems reasonable to just ask the local controller for controld_globals.dc_name, using that as the source of truth instead of broadcasting a ping.

• If it's NULL when there is in fact a DC somewhere, that should be a very temporary state, and "no DC" is still "true" from the local perspective.
• If it's set when the DC has gone offline or something, then similar to above.
• If it's possible for multiple nodes to transiently see themselves as DC, then that already complicates the "ping the DC and wait for a response" approach we use now. There's no obviously correct answer in that case.

All of this stems from dropping the mainloops in pcmk_cluster_queries.c. We replaced them with sync when the timeout is 0 in a6ec43e3. We replaced them with poll when the timeout is greater than 0 in 8771565f. The sync change is causing this problem. It's unclear if the poll change has caused or will cause any problems; that seems less likely, but it's possible.