restore APC hook for register serializer, fix #24

This PR restore the old ABI.
I also restore the return type from bool to int.
Thus an application (ex igbinary) build against APC will work, and serializer will be properly registered / listed.

At some time it could be needed to change this ABI (increasing APC_SERIALIZER_ABI), but I think it's a good idea to preserve it for some time (and wait for a major version).

Thanks for considering this.