cmd/vet: expand printf flags understood by %s and %q.
cmd/vet: expand printf flags understood by %s and %q.

Fixes issue 4580.

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/7002043

pombredanne/go-deleteme,tv42/old-go,tv42/old-go,gnanderson/go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,gnanderson/go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,apparentlymart/golang,dsriram/golang,tv42/old-go,apparentlymart/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,dsriram/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme