9p: exit when stdout or stderr is closed

[rakoo]

I often use 9p read long/lived/file | short-lived-process in my scripts. When short-lived-process exits, 9p doesn't see it until it tries to write again, which can be in an indeterminate time.

This PR polls stdout and stderr, assumes that a POLLERR means either was closed, and exits. It has been tested on linux only.

To reproduce:

$ 9p read acme/log | sed '/del/q'

sed should stop as soon as an acme window is closed, and that should inform 9p to stop reading the log. Currently it doesn't.