Add -flags flag to cmd/vet that causes it to describe its flags as JSON.

go vet's "-vettool" flag has been replaced with an environment
variable, GOVETTOOL, for two reasons:

  1) we need its value before flag processing,
     because we must run vet to discover its flags.

  2) users may change the env var to opt in/out of the new vet tool
     during the upcoming transition to vet based on the analysis API.

Change-Id: I5d8f90817623022f4170b88fab3c92c9b2fbdc37
Reviewed-on: https://go-review.googlesource.com/c/142617
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>