R1:fc950140b4c1

Blanket reject request which may have been poisoned by a "Proxy" header to mitigate the httpoxy vulnerability

Summary:
See accompanying discussion in T11359.

As far as I can tell we aren't vulnerable, but subprocesses could be (now, or in the future). Reject any request which may have a `Proxy:` header.

This will also do a false-positive reject if `HTTP_PROXY` is defined in the environment, but this is likely a misconfiguration (cURL does not read it). I'll provide guidance on this.

Test Plan:
- Made…