Due to a bug in the Darwin kernel, write(2) calls have a maximum size
of INT_MAX bytes.

Introduce a new compat function, clipped_write(), that only writes
at most INT_MAX bytes and returns the number of bytes written, as
a substitute for write(2), and allow platforms that need this to
enable it from the build mechanism with NEEDS_CLIPPED_WRITE.

Set it for Mac OS X by default.  It may be necessary to include this
function on Windows, too.
Signed-off-by: NFilipe Cabecinhas <filcab+git@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>