Impose a limit of 4k on HTTP headers and request lines

[djs55]

In case someone pipes large amounts of binary junk through the proxy, we impose a 4k limit on line length. Unfortunately we will send back a miscellaneous 500 server error rather than the correct 413 Entity too large, but this is better than nothing.

• https://github.com/mirage/mirage-channel/pull/25
• https://github.com/mirage/ocaml-cohttp/pull/612