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>