Auto merge of #1078 - johalun:so_reuseport_lb, r=alexcrichton

Add new FreeBSD socket option SO_REUSEPORT_LB.

FreeBSD 12, which is scheduled to be released soon, has a new socket option SO_REUSEPORT_LB.
From setsockopt man page:
     SO_REUSEPORT_LB allows completely duplicate bindings by multiple
     processes if they all set SO_REUSEPORT_LB before binding the port.
     Incoming TCP and UDP connections are distributed among the sharing
     processes based on a hash function of local port number, foreign IP
     address and port number. A maximum of 256 processes can share one socket.