Fix the `-DTHREADSAFE=OFF` build
This change avoids using the `(void)0` construct for some of the mutex
`#define`s, since that makes the "return type" of those "functions" to
be `void` instead of `int`.
runtime: move init/shutdown into the "runtime"
Provide a mechanism for system components to register for initialization
and shutdown of the libgit2 runtime.
