testing: introduce (*B).ReportAllocs()
Calling it will show memory allocation statistics for that
single benchmark (if -test.benchmem is not provided)

R=golang-dev, rsc, kevlar, bradfitz
CC=golang-dev
https://codereview.appspot.com/7027046
os: use SameFile in TestStartProcess
Fixes issue 4650.

R=golang-dev, bradfitz, alex.brainman
CC=golang-dev
https://codereview.appspot.com/7085048
cmd/godoc: when redirecting don't clear query string
so that http://golang.org/pkg/runtime?m=all works.

R=bradfitz
CC=golang-dev
https://codereview.appspot.com/7094046