cmd/vet: report uncalled functions in Printf %v

cmd/vet: report uncalled functions in Printf %v

Given, say, var f *os.File, a new vet check in CL 14122 diagnoses:

	fmt.Printf("%s\n", f.Name)
	fmt.Println(f.Name)

but not

	fmt.Printf("%v\n", f.Name)

In all three cases the error is that the argument should be f.Name().

Diagnosing Println but not Printf %v seems oddly inconsistent,
so I changed %v to have the check too. In fact, all verbs now have
the check except %p and %T.

Fixes Dave Cheney's confusion when trying to write an example
of the new vet check advertised in the Go 1.6 release notes.

Change-Id: I92fa6a7a1d5d9339a6a59ae4e587a254e633f500
Reviewed-on: https://go-review.googlesource.com/19101
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>