netops: init OpenSSL once under lock

We want to check whether the credentials callback is NULL, not whether
the payload is.

Bring together all of the OpenSSL initialization to
git_threads_init() so it's together and doesn't need locks.

Moving it here also gives us libssh2 thread safety (when built against
openssl).

OpenSSL's tests init everything in the main thread, so let's do that.

When using in a multithreaded context, OpenSSL needs to lock, and leaves
it up to application to provide said locks.

We were not doing this, and it's just luck that's kept us from crashing
up to now.

The OpenSSL init functions are not reentrant, which means that running
multiple fetches in parallel can cause us to crash.

Use a mutex to init OpenSSL, and since we're adding this extra checks,
init it only once.

Instead of using a sentinel empty value to detect the last commit, let's
check for when we get a NULL from popping the stack, which lets us know
when we're done.

The current code causes us to read uninitialized data, although only on
RHEL/CentOS 6 in release mode. This is a readability win overall.

By inserting in the right position, we can keep the vector sorted,
making entry insertion almost twice as fast.

More remote rename fixes