net: move deadline logic into pollServer
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