There were numerous small issues with test cases:

- some lacked the right source file header
- some were not portable at all
- some were using internal libzmq APIs (headers)

Solution: fixed and cleaned up.

And I'm on a reasonably sized laptop. I think allocating INT_MAX
memory is dangerous in a test case.

Solution: expose this as a context option. I've used ZMQ_MAX_MSGSZ
and documented it and implemented the API. However I don't know how
to get the parent context for a socket, so the code in zmq.cpp is
still unfinished.

Solution: rename socket option (and variables and files) from usefd
to use_fd.

Solution: rename socket option (and variables and files) from
pre_allocated_fd to usefd.

Solution: add new [set|get]sockopt ZMQ_PRE_ALLOCATED_FD to allow
users to let ZMQ use a pre-allocated file descriptor instead of
allocating a new one. Update [set|get]sockopt documentation and
test accordingly.

The main use case for this feature is a socket-activated systemd
service. For more information about this feature see:
http://0pointer.de/blog/projects/socket-activation.html

Solution: add test case