cmd/go: many improvements

* correct dependency calculations
* comment meaning of action fields
* new alias "std" like "all" but standard packages only
* add -o flag to 'go build'
* set up for parallel build (still serial)
* understand that import "C" depends on cgo, runtime/cgo

R=golang-dev, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/5502055
23 files changed