Add compat/fopen.c which returns NULL on attempt to open directory

Some systems do not fail as expected when fread et al. are called on
a directory stream. Replace fopen on such systems which will fail
when the supplied path is a directory.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>