psql: Fix logging output format

In normal interactive mode, psql's log messages accidentally got a
"psql:" prefix that was not supposed to be there.  This only happened
if there was no .psqlrc file being read, so it wasn't discovered for a
while.  Fix this by adding the appropriate logging format
configuration call in the right code path.

Discussion: https://www.postgresql.org/message-id/7586.1560540361@sss.pgh.pa.us

Author: petere

src/bin/psql/startup.c

@@ -437,6 +437,7 @@ main(int argc, char *argv[])
 	 */
 	else
 	{
+		pg_logging_config(PG_LOG_FLAG_TERSE);
 		connection_warnings(true);
 		if (!pset.quiet)
 			printf(_("Type \"help\" for help.\n\n"));