The header src/cc-compat.h defines portable format specifiers PRIuZ, PRIdZ, and PRIxZ. The original report highlighted the need to use these specifiers in examples/network/fetch.c.  For this commit, I checked all C source and header files not in deps/ and transitioned to the appropriate format specifier where appropriate.

Rather minimal change, but it's the kind of thing we should do.

This significantly reduces contention when many threads are trying to
read from the cache simultaneously.

This adds a `git__memset` routine that will not be optimized away
and updates the places where I memset() right before a free() call
to use it.

It is obviously quite a serious problem if this happens, but mutex
initialization can fail and we should detect it.  It's a bit like
a memory allocation failure, in that you're probably pretty screwed
if this occurs, but at least we'll catch it.

By zeroing out the memory when we free larger objects (i.e. those
that serve as collections of other data, such as repos, odb, refdb),
I'm hoping that it will be easier for libgit2 bindings to find
errors in their object management code.

This adds docs for the cache control options to git_libgit2_opts
and also tweaks the cache code so that if the cache is disabled,
then the next time we attempt to insert something into the cache
in question, we will actually clear any old cached objects.