Since it doesn't make sense to make the disk access stuff
portable *AND* public (that's a job for each application
imo), we can take a shortcut and just support unixy stuff
for now and get away with coding most of it as macros.

Since we go with an internal API for starters and only
provide higher-level API's to the libgit users, we'll be
ok with this approach.

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>