Update #4434.
The proposal attempts to reduce the number of places where fd,{r,w}deadline is checked and updated in preparation for issue 4434. In doing so the deadline logic is simplified by letting the pollster return errTimeout from netFD.Wait{Read,Write} as part of the wakeup logic.
The behaviour of setting n = 0 has been restored to match rev 2a55e349097f, which was the previous change to fd_unix.go before CL 6851096.
R=jsing, bradfitz, mikioh.mikioh, rsc
CC=fullung, golang-dev
https://golang.org/cl/6850110
A fix similar to CL 6859043 was effective in resolving the intermittent failure.
Fixes#4423.
R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6854102