Thanks to Dustin Sallings for exposing the most frustrating
bug ever, and for providing repro cases (which formed the
basis of the new tests in this CL), and to Dave Cheney and
Dmitry Vyukov for help debugging and fixing.

This CL depends on submited pollster CLs ffd1e075c260 (Unix)
and 14b544194509 (Windows), as well as unsubmitted 6852085.
Some operating systems (OpenBSD, NetBSD, ?) may still require
more pollster work, fixing races (Issue 4434 and
http://goo.gl/JXB6W).

Tested on linux-amd64 and darwin-amd64, both with GOMAXPROCS 1
and 4 (all combinations of which previously failed differently)

Fixes #4191
Update #4434 (related fallout from this bug)

R=dave, bradfitz, dsallings, rsc, fullung
CC=golang-dev
https://golang.org/cl/6851061