dgram: implicit binds should be exclusive

(re)-PR of fix for https://github.com/iojs/io.js/pull/279

Passes make test.

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.

Since implicit binds become exclusive, implicit/client dgram sockets can now be used with cluster on Windows. Before, neither explicit nor implicitly bound sockets could be used, causing dgram to be completely unsupported with cluster on Windows. After this change, they become half supported.

PR-URL: https://github.com/joyent/node/pull/8643 Reviewed-by: Bert Belder bertbelder@gmail.com