strings: define byteBitmap.isSet

LGTM=dave
R=golang-codereviews, bradfitz, dave
CC=golang-codereviews
https://codereview.appspot.com/109090048
runtime: remove obsolete afterprologue check
Afterprologue check was required when did not know
about return arguments of functions and/or they were not zeroed.
Now 100% precision is required for stacks due to stack copying,
so it must work w/o afterprologue one way or another.
I can limit this change for 1.3 to merely adding a TODO,
but this check is super confusing so I don't want this knowledge to get lost.

LGTM=rsc
R=golang-codereviews, gobot, rsc, khr
CC=golang-codereviews, khr, rsc
https://codereview.appspot.com/96580045
net: simplify code
Single-case select with a non-nil channel is pointless.

LGTM=mikioh.mikioh
R=mikioh.mikioh
CC=golang-codereviews
https://codereview.appspot.com/103920044
sync: detect incorrect usages of RWMutex
Fixes issue 7858.

LGTM=ruiu
R=ruiu
CC=golang-codereviews
https://codereview.appspot.com/92720045