Add verbosity to pg_basebackup for sync

This is useful to know when the data copy has been finished.  The
current situation can be confusing for users as the last message is
"waiting for background process to finish streaming", so it looks like
this is taking time but the final sync is instead.

Author: Jeff Janes
Discussion: https://postgr.es/m/CAMkU=1ypeoMJ=tFBG8vP13sxEtXd4Pm_x1SqsJdW_RvzpcvN=A@mail.gmail.com

src/bin/pg_basebackup/pg_basebackup.c

@@ -2112,6 +2112,9 @@ BaseBackup(void)
 	 */
 	if (do_sync)
 	{
+		if (verbose)
+			fprintf(stderr,
+					_("%s: syncing data to disk ...\n"), progname);
 		if (format == 't')
 		{
 			if (strcmp(basedir, "-") != 0)