fmt's %d, %x, and %X all accept pointer arguments. However, in cmd/vet's
printVerbs table, they were defined as if they did not accept pointer
arguments.

This inconsistency with fmt did not manifest to users since the vet
codebase worked around it. In particular, pointer arguments were usually
allowed for verbs that accepted integers, as the *types.Pointer argument
type case read the following:

	t&(argInt|argPointer) != 0

As a result, using the %q verb with a pointer resulted in a bug in
cmd/vet:

	$ go run f.go
	%!q(*int=0xc000014140)
	$ go vet f.go
	[no warning]

As documented, fmt's %q verb only accepts runes (integers), strings, and
byte slices. It should not accept pointers, and it does not. But since
vet mixed integers and pointers, it wasn't properly warning about the
misuse of fmt.

This patch surfaced another bug with fmt.Printf("%p", nil):

	$ go run f.go
	%!p(<nil>)
	$ go vet f.go
	[no warning]

As documented, fmt's %p verb only accepts pointers, and untyped nil is
not a valid pointer. But vet did not warn about it, which is another
inconsistency with fmt's documented rules. Fix that too, with a test,
also getting rid of the TODO associated with the code.

As a result of those changes, fix a wrong use of the fmt format verbs in
the standard library, now correctly spotted by vet.

Fixes #25233.

Change-Id: Id0ad31fbc25adfe1c46c6b6879b8d02b23633b3a
Reviewed-on: https://go-review.googlesource.com/111284
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>