os: delete Exec, NewFile takes uintptr, rename ShellExpand, doc fixes

Delete O_NDELAY, O_NONBLOCK, O_NOCTTY, O_ASYNC.

Clean up some docs.

Rename ShellExpand -> ExpandEnv.

Make NewFile take a uintptr; change File.Fd to return one.
(for API compatibility between Unix and Windows)

Fixes #2947

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5655045