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>

config.mak.uname

@@ -233,6 +233,7 @@ ifeq ($(uname_S),OpenBSD)
 	HAVE_BSD_SYSCTL = YesPlease
 	HAVE_BSD_KERN_PROC_SYSCTL = YesPlease
 	PROCFS_EXECUTABLE_PATH = /proc/curproc/file
+	FREAD_READS_DIRECTORIES = UnfortunatelyYes
 endif
 ifeq ($(uname_S),MirBSD)
 	NO_STRCASESTR = YesPlease