More synchronous replication tweaks.

SyncRepRequested() must check not only the value of the
synchronous_replication GUC but also whether max_wal_senders > 0.
Otherwise, we might end up waiting for sync rep even when there's no
possibility of a standby ever managing to connect.  There are some
existing cross-checks to prevent this, but they're not quite sufficient:
the user can start the server with max_wal_senders=0,
synchronous_standby_names='', and synchronous_replication=off and then
subsequent make synchronous_standby_names not empty using pg_ctl reload,
and then SET synchronous_standby=on, leading to an indefinite hang.

Along the way, rename the global variable for the synchronous_replication
GUC to match the name of the GUC itself, for clarity.

Report by Fujii Masao, though I didn't use his patch.

src/backend/utils/misc/guc.c

@@ -759,7 +759,7 @@ static struct config_bool ConfigureNamesBool[] =
 			gettext_noop("Requests synchronous replication."),
 			NULL
 		},
-		&sync_rep_mode,
+		&synchronous_replication,
 		false, NULL, NULL
 	},
 	{