git_odb_backend_one_packfile() allows us to create an ODB backend out
of an .idx file.

Go through each backend and list every objects that exists in
them. This allows fsck-like uses.

There are three changes here:
- correctly propogate error code from failed object lookups
- make zlib inflate use our allocators
- add OID to notfound error in ODB lookups

This fixes all the warnings on win64 except those in deps, which
come from the regex code.

Also cleaned up some previously converted code that still had
little things to polish.

This migrates odb.c, odb_loose.c, odb_pack.c and pack.c to
the new style of error handling.  Also got the unix and win32
versions of map.c.  There are some minor changes to other
files but no others were completely converted.

This also contains an update to filebuf so that a zeroed out
filebuf will not think that the fd (== 0) is actually open
(and inadvertently call close() on fd 0 if cleaned up).

Lastly, this was built and tested on win32 and contains a
bunch of fixes for the win32 build which was pretty broken.

Yes, this is error handling solely for `refs.c`, but some of the
abstractions leak all ofer the code base.

It turns out that commit 31e9cfc4cbcaf1b38cdd3dbe3282a8f57e5366a5
did not fix the GIT_USUSED behavior on all platforms.  This commit
walks through and really cleans things up more thoroughly, getting
rid of the unnecessary stuff.

To remove the use of some GIT_UNUSED, I ended up adding a couple
of new iterators for hashtables that allow you to iterator just
over keys or just over values.

In making this change, I found a bug in the clar tests (where we
were doing *count++ but meant to do (*count)++ to increment the
value).  I fixed that but then found the test failing because it
was not really using an empty repo.  So, I took some of the code
that I wrote for iterator testing and moved it to clar_helpers.c,
then made use of that to make it easier to open fixtures on a
per test basis even within a single test file.

This is legacy compat stuff for when `deflateBound` is not defined, but
we're not embedding zlib and that function is always available. Kill
that with fire.