wrapper: add git_mkdtemp()

wrapper: add git_mkdtemp()

Extend git_mkstemps_mode() to optionally call mkdir(2) instead of
open(2), then use that ability to create a mkdtemp(3) replacement,
git_mkdtemp().  We'll start using it in the next commit.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>