config.mak.uname: set FREAD_READS_DIRECTORIES for Linux and FreeBSD

config.mak.uname: set FREAD_READS_DIRECTORIES for Linux and FreeBSD

This variable is added [1] with the assumption that on a sane system,
fopen(<dir>, "r") should return NULL. Linux and FreeBSD do not meet this
expectation while at least Windows and AIX do. Let's make sure they
behave the same way.

I only tested one version on Linux (4.7.0 with glibc 2.22) and
FreeBSD (11.0) but since GNU/kFreeBSD is fbsd kernel with gnu userspace,
I'm pretty sure it shares the same problem.

[1] cba22528fa (Add compat/fopen.c which returns NULL on attempt to open
    directory - 2008-02-08)

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>