cluster: don't assert if worker has no handles

For shared (non-round-robin) handles (UDP, and all handles on Windows),
cluster must not assume that workers are completely symetric, in that if
one listens or binds to a port, every worker listens or binds to that
port.
dgram: implicit binds should be exclusive

Server sockets should be shared by default, and client sockets should be
exclusive by default. For net/TCP, this is how it is, for dgram/UDP, its
a little less clear what a client socket is, but a socket that is
auto-bound during a dgram.send() is not usefully shared among cluster
workers, any more than an outgoing TCP connection would be usefully
shared.