net: more refactoring in preparation for runtime integrated pollster
Move pollServer from fd_unix.go to fd_poll_unix.go.
Add pollServerInit(*NetFD) to allow custom initialization.
Add pollServer.Close(*NetFD) to allow custom finalization.
Move setDeadline() to fd_poll_unix.go to allow custom handling of deadlines.
Move newPollServer() to fd_poll_unix.go to allow custom initialization.
No logical code changes.
The next step will be to turn off fd_poll_unix.go for some platform
(I have changes for darwin/linux) and redirect it into runtime. See:
https://codereview.appspot.com/7569043/diff/2001/src/pkg/net/fd_poll_runtime.go

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7513045