readline: fix calling constructor without new

[rlidwka]

This works:

$ iojs -e 'new (require("readline").Interface)({ input: process.stdin, output: process.stdout })'

This doesn't:

$ iojs -e 'require("readline").Interface({ input: process.stdin, output: process.stdout })'
readline.js:98
    input.on('data', ondata);
          ^
TypeError: undefined is not a function
    at new Interface (readline.js:98:11)
    at Object.Interface (readline.js:29:12)
    at [eval]:1:21
    at Object.exports.runInThisContext (vm.js:54:17)
    at Object.<anonymous> ([eval]-wrapper:6:22)
    at Module._compile (module.js:446:26)
    at evalScript (node.js:413:25)
    at startup (node.js:72:7)
    at node.js:799:3

The reason is that option object detection here relies on a number of arguments passed to the constructor.

But instanceof guard here always makes it four arguments.

---

So the issue happens when usual instanceof check is combined with logic based on arguments.length.

There are two places in io.js core when it matters. Second one is in buffer.js and it's already handled correctly.

[bnoordhuis]

Suggestion: use const to keep it lexically scoped.

[rlidwka]

changed to const in the next commit

[bnoordhuis]

LGTM with a suggestion.

[bnoordhuis]

Cheers @rlidwka, landed in f0bf6bb.