cmd/go, testing: add TestMain support

Fixes issue 8202.

LGTM=r, bradfitz
R=r, josharian, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/148770043
runtime: add runtime· prefix to some static variables

Pure renaming. This will make an upcoming CL have smaller diffs.

LGTM=dvyukov, iant
R=iant, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/142280043
net/http: document server recovering panics

Fixes issue 8594.

LGTM=bradfitz
R=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/145760043