This CL gives goinstall the ability to build commands,
not just packages.

"goinstall foo.googlecode.com/hg/bar" will build the command named
"bar" and install it to GOBIN. "goinstall ." will use the name of the
local directory as the command name.

R=rsc, niemeyer
CC=golang-dev
https://golang.org/cl/4426045