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/replication/syncrep.c

@@ -62,7 +62,7 @@
 #include "utils/ps_status.h"
 
 /* User-settable parameters for sync rep */
-bool	sync_rep_mode = false;			/* Only set in user backends */
+bool	synchronous_replication = false;		/* Only set in user backends */
 char 	*SyncRepStandbyNames;
 
 static bool sync_standbys_defined = false;	/* Is there at least one name? */