Add stub support for TCP_WINDOW_CLAMP

[hbhasker]

Netstack does not support Window clamping but looks like py-amqp relies on the getsockopt to not return an error

See: #1441 (comment)

We can stub this out for now and implement clamping if required at a later time.

TCP_WINDOW_CLAMP
Bound the size of the advertised window to this value. The kernel imposes a minimum size of SOCK_MIN_RCVBUF/2. This option should not be used in code intended to be portable.