Add --disallow-untyped-functions

[ddfisher]

This is something we want to add sooner or later, so I figured I'd just add it now that someone's explicitly asked for it. This catches ~10 incomplete signatures in builtins.pyi and typing.pyi, which would need to be fixed up in a separate typeshed PR.

Fixes #1282.

[gvanrossum]

• Is there no way to have a unit test for this? It's not very complex but it could still easily be broken by other changes.
• Could you fix typeshed (at least builtins and typing) before merging this?
• I think the meaning of the option would be clearer if it was disallow-untyped-defs.

[ddfisher]

• Not sure I see this being done for other flags currently, but it seems like a good idea -- I'll see how difficult it is.
• Looks like you synced typeshed. Thanks!
• I agree. Changed!

[gvanrossum]

Let me know about the unit tests.

[ddfisher]

Added unit tests!