cmd/gc: fix imported and not used error for import .

Fixes issues 6420.

R=ken2
CC=golang-dev
https://codereview.appspot.com/13703044
test/run: print command line for running an individual test

Fixes issue 5087.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13812043