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