pg_upgrade: call 'postgres' binary to get data directory location

This matches the binary 'pg_ctl' calls.  Previously we called the
'postmaster'.

Report by Christoph Berg

contrib/pg_upgrade/option.c

@@ -415,7 +415,7 @@ adjust_data_dir(ClusterInfo *cluster)
 	 * so this might fail --- only works for PG 9.2+.   If this fails,
 	 * pg_upgrade will fail anyway because the data files will not be found.
 	 */
-	snprintf(cmd, sizeof(cmd), "\"%s/postmaster\" -D \"%s\" -C data_directory",
+	snprintf(cmd, sizeof(cmd), "\"%s/postgres\" -D \"%s\" -C data_directory",
 			 cluster->bindir, cluster->pgconfig);
 
 	if ((output = popen(cmd, "r")) == NULL ||