channel: truncate twrite messages based on msize

While there are a few problems around handling of msize, the easiest to address and, arguably, the most problematic is that of Twrite. We now truncate Twrite.Data to the correct length if it will overflow the msize limit negotiated on the session. ErrShortWrite is returned by the `Session.Write` method if written data is truncated. In addition, we now reject incoming messages from `ReadFcall` that overflow the msize. Such messages are probably terminal in practice, but can be detected with the `Overflow` function. Tread is also handled accordingly, such that the Count field will be rewritten such that the response doesn't overflow the msize. Signed-off-by: Stephen J Day <stephen.day@docker.com>

p9p: use new context package throughout

Signed-off-by: Stephen J Day <stephen.day@docker.com>

Signed-off-by: Stephen J Day <stephen.day@docker.com>

Signed-off-by: Stephen J Day <stephen.day@docker.com>