cmd/yacc: spring cleaning for units.y
cmd/yacc: spring cleaning for units.y

* sort imports
* use runtime.GOROOT
* fix some typos

R=golang-dev, dave, rsc
CC=golang-dev
http://codereview.appspot.com/5987054

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang