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 dominikh#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 dominikh#466