lib,src: add unix socket getsockname/getpeername

The implementation is a minor API change in that socket.address() now
returns a `{ address: '/path/to/socket' }` object, like it does for TCP
and UDP sockets.  Before this commit, it returned `socket._pipeName`,
which is a string when present.

Change common.PIPE on Windows from '\\\\.\\pipe\\libuv-test' to
'\\\\?\\pipe\\libuv-test'.  Windows converts the '.' to a '?' when
creating a named pipe, meaning that common.PIPE didn't match the
result from NtQueryInformationFile().

Fixes: https://github.com/nodejs/node/issues/954
PR-URL: https://github.com/nodejs/node/pull/956
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

test/parallel/test-http-unix-socket.js

@@ -9,6 +9,7 @@ var headers_ok = false;
 var body_ok = false;
 
 var server = http.createServer(function(req, res) {
+  assert.equal(req.socket.address().address, common.PIPE);
   res.writeHead(200, {
     'Content-Type': 'text/plain',
     'Connection': 'close'
@@ -19,6 +20,7 @@ var server = http.createServer(function(req, res) {
 });
 
 server.listen(common.PIPE, function() {
+  assert.equal(server.address().address, common.PIPE);
 
   var options = {
     socketPath: common.PIPE,