cmd/vet: bring in sigchanyzer to report unbuffered channels to signal.Notify

cmd/vet: bring in sigchanyzer to report unbuffered channels to signal.Notify

Brings in the static analyzer "sigchanyzer", that we created at
Orijtech, Inc, and already submitted in CL 274352, as

    golang.org/x/tools/go/analysis/passes/sigchanyzer

and add it to cmd/vet as one of the passes.

Fixes #9399

Change-Id: I83708b8ea5ca8ede5ee31efab55cbce7419434ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/299532
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Bryan C. Mills <bcmills@google.com>