struct PQWalReceiverFunctions: use designated initializers

We now require that compilers support this, and it makes the code easier
to trace, so change it.  I'm fixated on this particular struct because
I've had to navigate around it a number of times, but there are others
elsewhere that could use the same treatment.

Discussion: https://postgr.es/m/20220810140300.ixhbmm4svo5yypv6@alvherre.pgsql

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -82,21 +82,21 @@ static WalRcvExecResult *libpqrcv_exec(WalReceiverConn *conn,
 static void libpqrcv_disconnect(WalReceiverConn *conn);
 
 static WalReceiverFunctionsType PQWalReceiverFunctions = {
-	libpqrcv_connect,
+	.walrcv_connect = libpqrcv_connect,
-	libpqrcv_check_conninfo,
+	.walrcv_check_conninfo = libpqrcv_check_conninfo,
-	libpqrcv_get_conninfo,
+	.walrcv_get_conninfo = libpqrcv_get_conninfo,
-	libpqrcv_get_senderinfo,
+	.walrcv_get_senderinfo = libpqrcv_get_senderinfo,
-	libpqrcv_identify_system,
+	.walrcv_identify_system = libpqrcv_identify_system,
-	libpqrcv_server_version,
+	.walrcv_server_version = libpqrcv_server_version,
-	libpqrcv_readtimelinehistoryfile,
+	.walrcv_readtimelinehistoryfile = libpqrcv_readtimelinehistoryfile,
-	libpqrcv_startstreaming,
+	.walrcv_startstreaming = libpqrcv_startstreaming,
-	libpqrcv_endstreaming,
+	.walrcv_endstreaming = libpqrcv_endstreaming,
-	libpqrcv_receive,
+	.walrcv_receive = libpqrcv_receive,
-	libpqrcv_send,
+	.walrcv_send = libpqrcv_send,
-	libpqrcv_create_slot,
+	.walrcv_create_slot = libpqrcv_create_slot,
-	libpqrcv_get_backend_pid,
+	.walrcv_get_backend_pid = libpqrcv_get_backend_pid,
-	libpqrcv_exec,
+	.walrcv_exec = libpqrcv_exec,
-	libpqrcv_disconnect
+	.walrcv_disconnect = libpqrcv_disconnect
 };
 
 /* Prototypes for private functions */