Provide a callback for certificate validation

Call the user if they want to in order to check the certificate/host info. Always call but provide our decision in the HTTP case. The user gets the last word if the callback is set.

- [x] Implement for HTTP
  - [x] Tests for HTTP
- [x] Implement for SSH
  - [x] Tests for SSH
- [x] Implement for WinHTTP
  - [x] Tests for WinHTTP
Yeah, I know, I feel like the queue is by far the most expensive part of our CI, but it's also the one we have the least control over.  (Or is it?  Do private travis builds have a different, faster queue?  Can we pay cash money but still have open build reports?)

Still, it's CPU that we weren't taking advantage of before.  We might as well use it.

Now - parallelising clar would be an interesting task.