pg_regress: Add application name setting

Set the PGAPPNAME environment variable in pg_regress so that it
identifies itself as such instead of "psql".

src/test/regress/pg_regress.c

@@ -691,6 +691,8 @@ initialize_environment(void)
 {
 	char	   *tmp;
 
+	putenv("PGAPPNAME=pg_regress");
+
 	if (nolocale)
 	{
 		/*