SA9009: flag defer foo() where foo returns a closure

[arp242]

Add a new checker to flag cases where a deferred function returns exactly one function, and that function isn't called. For example:

func f() func() {
	// Do stuff.
	return func() {
		// Do stuff
	}
}

func main() {
	defer f() // Error: should be f()()
}

As mentioned in #466, there's a change for false positives. I ran the check on Go stdlib, all my own code, and about 200 popular Go packages, and haven't found any false positives (and one match: grpc/grpc-go#7270)

Fixes #466

[arp242]

I ran the same script for #1545 with this check; found three more wrong uses of defer, but no false positives.