runtime: remove debugging knob to turn off preemption

It's still easy to turn off, but the builders are happy.
Also document.

R=golang-dev, iant, dvyukov
CC=golang-dev
https://codereview.appspot.com/12371043

Committer: Russ Cox <rsc@golang.org>
compress/bzip2: support concatenated files

While we're here, add a test for the same functionality in gzip,
which was already implemented, and add bzip2 CRC checks.

Fixes issue 5772.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/12387044