runtime/race: switch to explicit race context instead of goroutine id's
Removes limit on maximum number of goroutines ever existed.
code.google.com/p/goexecutor tests now pass successfully.
Also slightly improves performance.
Before: $ time ./flate.test -test.short
real	0m9.314s
After:  $ time ./flate.test -test.short
real	0m8.958s
Fixes issue 4286.
The runtime is built from llvm rev 174312.

R=rsc
CC=golang-dev
https://codereview.appspot.com/7218044
doc/articles/error_handling: no more os.Error

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/7304053
exp/cookiejar: infrastructure for upcoming implementation

This CL is the first of a handful of CLs which will provide
the implementation of cookiejar. It contains several helper
functions and the skeleton of Cookies and SetCookies.

Proper host name handling requires the ToASCII transformation
from package idna which currently lives in the go.net
subrepo. This CL thus contains just a TODO for this issue.

R=nigeltao, rsc, bradfitz
CC=golang-dev
https://codereview.appspot.com/7287046

Committer: Nigel Tao <nigeltao@golang.org>