can_write() was saying yes in situations where write() would overflow the
internal buffer. This patch adds a has_attached_peer() helper to make it
easier to understand what's going on in these functions.
Make the Socket functions take a FileDescriptor& rather than a socket role
throughout the code. Also change threads to block on a FileDescriptor,
rather than either an fd index or a Socket.
If connect() is called on a non-blocking socket, it will "fail" immediately
with -EINPROGRESS. After that, you select() on the socket and wait for it to
become writable.
