Don't share dgram sockets that are implicitly bound

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.

See https://github.com/joyent/node/pull/8027#issuecomment-60817257

/to @asilvas @bnoordhuis  @rmg

Test triggers a pre-existing bug, so is based on https://github.com/joyent/node/pull/8642.