goinstall: error out with paths that end with '/'
goinstall: error out with paths that end with '/'

R=adg, rsc, tarmigan+golang
CC=golang-dev
http://codereview.appspot.com/4807048

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go