node-zsock-async: open Unix Doman Sockets inside illumos zones

This is an asynchronous version of the node-zsock add-on. As for why async matters here, see the node-spawn-async module.

This add-on allows programs running in the global zone of illumos- and Solaris-based systems (such as SmartOS or OmniOS) to securely create servers listening on a Unix Domain Socket bound inside a local zone. The idea is that an agent running in the global zone securely binds a UDS to a well-known path inside the local zone and starts an HTTP server. Components running inside the zone can make HTTP requests over the socket, and the global zone agent always knows which zone is talking to it.

For an example of using this add-on, see examples/basic.js.