global: adjust init count under lock

[pks-t]

Our global initialization functions git_libgit2_init() and
git_libgit2_shutdown() both adjust a global init counter to determine
whether we are the first respectively last user of libgit2. On
Unix-systems do not do so under lock, though, which opens the
possibility of a race between these two functions:

Thread 1                            Thread 2
                        git__n_inits = 0;
git_libgit2_init();
git_atomic_inc(&git__n_inits);
/* git__n_inits == 1 */
                                    git_libgit2_shutdown();
                                    if (git_atomic_dec(&git__n_inits) != 0)
                                        /* git__n_inits == 0, no early exit here */
                                    pthread_mutex_lock(&_init_mutex);
                                    shutdown_common();
                                    pthread_mutex_unlock(&_init_mutex);
pthread_mutex_lock(&_init_mutex);
init_once();
pthread_mutex_unlock(&_init_mutex);

So we can end up in a situation where we try to shutdown shared data
structures before they have been initialized.

Fix the race by always locking _init_mutex before incrementing or
decrementing git__n_inits.