cmd/vet: print help to stdout only
cmd/vet: print help to stdout only

Previously help for flags was printed to stderr.
This CL makes all the output to be printed to stdout.

Updates golang/go#31885

Change-Id: If95edeccd79581326502dd5c7fc2b49d8f160be7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/175900
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>
Run-TryBot: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools