net: fix concurrent Accept on windows
Runtime netpoll supports at most one read waiter
and at most one write waiter. It's responsibility
of net package to ensure that. Currently windows
implementation allows more than one waiter in Accept.
It leads to "fatal error: netpollblock: double wait".

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12400045
runtime: remove dead code
Remove dead code related to allocation of type metadata with SysAlloc.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12311045
runtime: remove unused scheduler knob
Blockingsyscall was used in net package on windows,
it's not used anymore.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12436043