Remove socket option SO_REUSEPORT.

It could be made optional in the future, but should not be on by
default.
Increment refcount for messages before they are dispatched.

Otherwise, if user code tries to incref then free the message it will
free non-owned memory.  This ensures the message is not freed until
the handler returns (as long as incref/free calls are well-matched.)
Remove some generated files from the dist.