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