lib9: add mktempdir, removeall, runprog

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7523043
cmd/vet: isolate the type checking code into a separate file
We can enable/disable type checking with a build tag.
Should simplify cutting the go1.1 distribution free of go/types.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7482045