runtime: convert parfor to Go

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews, khr
https://codereview.appspot.com/132100043
runtime: convert sigqueue to Go

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews, khr
https://codereview.appspot.com/132090043