config.mak.uname: OpenBSD uses BSD semantics with fread for directories

this "fixes" test 23 (proper error on directory "files") from t1308 MirBSD likely also affected but this was only tested with OpenBSD and therefore this specific change only affects that platform the optional 'configure' sets this automatically (tested with 6.1 to 6.4) but considering this is a legacy feature it is likely that it affected all old versions and is probably what most users had been using as a workaround Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>