Add ZMQ_MAX_SOCKETS_MAX to zmq_ctx_get()

The new options allows querying the maximum allowed number of sockets.
This is system dependent and cannot be encoded in the include file as a
preprocessor macro: for ZMQ_USE_SELECT, this depends on the FD_SETSIZE
macro at time of library compilation, not at time of include file use.
Fix a strict-aliasing with type-punning

Fixes #880
Revert "add proxy_chain, a multi proxies chaining in the same thread feature"

This reverts commit bc7441f5176589ad9a34dc2bde6b91d78a44e3e0.
Revert "add a proxy hook"

This reverts commit 9ae6a91fadb96fd48038fde04cc3d4b61b49a8a1.
fix revert
Merge pull request #891 from lalebarde/master

Reverse zmq_proxy_chain and zmq_proxy_hook
Merge pull request #890 from brunoqc/880

Fix a strict-aliasing with type-punning
Merge pull request #889 from olafmandel/MAX_SOCKETS_MAX

Add ZMQ_MAX_SOCKETS_MAX to zmq_ctx_get()