OSX has its own cryptographic library, let's make use of it instead of
calling out to OpenSSL.

Increasingly there are a number of components that want to do some
cleanup at global shutdown time (at least if there are not going
to be memory leaks).  This creates a very simple system of shutdown
hooks that will be invoked by git_threads_shutdown.  Right now, the
maximum number of hooks is hardcoded, but since adding a hook is
not a public API, it should be fine and I thought it was better to
start off with really simple code.

Thread safety for the refdb_fs

This loads SRWLock APIs at runtime and in their absence (i.e. on
Windows before Vista) falls back on a regular CRITICAL_SECTION
that will not permit concurrent readers.

This reverts commit efe7fad6c96a3d6197a218aeaa561ec676794499, except for
the indentation fixes.

Along with that, fix indentation in tests-clar/object/raw/hash.c