R1:ffdc082852fe

Add a wide range of HTTP-request-based setup checks

Summary:
Ref T11553. With some regularity, users make various configuration mistakes which we can detect by making a request to ourselves.

I use a magical header to make this request because we want to test everything else (parameters, path).

- Fixes T4854, probably. Tries to detect mod_pagespeed by looking for a header. This is a documentation-based "fix", I didn't actually install mod_pagespeed or formally test this.
- Fixes T6866. We now test…