Add #ifdef custom_compare_ext_default to accomodate newer OCaml.
Reflect newly added zmq_ctx_* function family in the binding.
Upgrade deprecated APIs.

ZMQ_IPV4ONLY was replaced with ZMQ_IPV6.
ZMQ_DELAY_ATTACH_ON_CONNECT was replaced with ZMQ_IMMEDIATE.
Implement all missing getsockopt/setsockopt options for 4.0.
Wrap zmq_z85_* family of functions.
Wrap zmq_curve_* family of functions.