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/include/replication/syncrep.h

@@ -19,7 +19,8 @@
 #include "storage/spin.h"
 #include "utils/guc.h"
 
-#define SyncRepRequested()				(sync_rep_mode)
+#define SyncRepRequested() \
+	(synchronous_replication && max_wal_senders > 0)
 
 /* syncRepState */
 #define SYNC_REP_NOT_WAITING		0
@@ -28,7 +29,7 @@
 #define SYNC_REP_MUST_DISCONNECT	3
 
 /* user-settable parameters for synchronous replication */
-extern bool sync_rep_mode;
+extern bool synchronous_replication;
 extern char *SyncRepStandbyNames;
 
 /* called by user backend */