um: Error handling fixes in vector drivers

With the addition of bess support which uses connection
oriented SEQPACKET sockets the vector routines can now
encounter a "remote end closed the connection" scenario.

This adds handling code to detect it in the TX path and
the legacy RX path. There is no way to detect it in the
vector RX path because that can legitimately return 0
even if the remote end has not closed the connection. As
a result the detection is delayed until the first TX
event after the close.

Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>