Make sock.connected much more robust by using socket.poll

[zachriggle]

The current code for sock.can_recv_raw and sock.connected_raw are a bit lacking, and only show what we know about the socket.

Specifically,

• can_recv_raw() will return True if the socket has closed
  • An attempt to recv will immediately throw EOFError
  • This is undesirable and incorrect, per the documentation on tube.can_recv
• connected() only knows about the internal state
  • If the remote side disconnects, and while sock.connected(): time.sleep(1) will never return
  • This is undesirable and incorrect, since we can't detect disconnections

This adds a bit of code using socket.poll for connected_raw(), and recv(..., MSG_PEEK) for can_recv_raw().

[zachriggle]

For posterity, here's a simple test:

>>> l=listen()
[x] Trying to bind to 0.0.0.0 on port 0
[x] Trying to bind to 0.0.0.0 on port 0: Trying 0.0.0.0
[+] Trying to bind to 0.0.0.0 on port 0: Done
[x] Waiting for connections on 0.0.0.0:55361
>>> r=remote('localhost', l.lport)
[x] Opening connection to localhost on port 55361
[x] Opening connection to localhost on port 55361: Trying ::1
[x] Opening connection to localhost on port 55361: Trying 127.0.0.1
[+] Opening connection to localhost on port 55361: Done
[+] Waiting for connections on 0.0.0.0:55361: Got connection from 127.0.0.1 on port 55363
>>> l.close()
[*] Closed connection to 127.0.0.1 port 55363
>>> r.wait_for_close() #<--- Never returns, before this pull request

[TethysSvensson]

LGTM