If its changed to TypeError, should be changed in other sig-checking functions, here and in src/node.js, to keep consistency.

Definition of lookupSignal above suffers from oddity of allowing ANY constants, not just signals, to be permitted, as does the definition of ChildProcess.prototype.kill.

It also allows any numbers, but process.kill() and ChildProcess.prototype.kill only allows 0 and string constants. It looks like numeric 15 will pass lookupSignal() type check, then be delivered to child.kill(), where it will throw.

I'd suggest that the logic be synced with isSignal() in https://github.com/joyent/node/blob/master/src/node.js#L584. I could extend #6365 to these signal name checks, if you'd like, so that its the same check everywhere.