net: handle hangup in read on Plan 9

net: handle hangup in read on Plan 9

On Plan 9, when closing a TCP connection, we
write the "hangup" string to the TCP ctl file.

The next read on the TCP data file will return
an error like "/net/tcp/18/data: Hangup", while
in Go, we expect to return io.EOF.

This change makes Read to return io.EOF when
an error string containing "Hangup" is returned.

Change-Id: I3f71ed543704190b441cac4787488a77f46d88a1
Reviewed-on: https://go-review.googlesource.com/22149
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>