If we aren't connected, return `Eof

This is expected behaviour after the other end has disconnected.

The case where {read,write] is called before a connection has been
accepted should be handled by blocking (not in this patch)

Signed-off-by: David Scott <dave.scott@citrix.com>
Lwt_io: on `Eof from the FLOW, return 0 bytes read or written

This matches the behaviour of the file descriptors that Lwt_io is
usually used over.

Signed-off-by: David Scott <dave.scott@citrix.com>
Implement Lwt_io.close on the input side of a vchan to close the connection.

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #29 from djs55/test

Partial implementation of close