cmd/vet: %b is a valid floating-point format.
Also add a report about "invalid type" from gotype, if -v is set.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7420045
all: fix a few more printf arg bugs found by go vet

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7413045