Windows: redirect f[re]open("/dev/null") to f[re]open("nul")

Windows: redirect f[re]open("/dev/null") to f[re]open("nul")

On Windows, the equivalent of "/dev/null" is "nul". This implements
compatibility wrappers around fopen() and freopen() that check for this
particular file name.

The new tests exercise code paths where this is relevant.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>