rproxy: remove forwarded headers spelt with underscores from an untrusted peer (closes #334)

For a peer outside `SWRP_TRUSTED_PROXIES`, `rproxy` now reads each header name with `_` as `-` and in any letter case, and removes it when it starts with `X-Forwarded-` or is `Forwarded` or `X-Real-IP`. An app behind CGI or FastCGI reads `X_Forwarded_Ssl` as `HTTP_X_FORWARDED_SSL`, the same as `X-Forwarded-Ssl`, so a client connecting directly could still make it take a plain request for a secure one. A trusted peer's headers pass on unchanged. `SPEC.md` says so.

Model: opus-5-5

SPEC.md

@@ -2326,7 +2326,8 @@ but for these headers:
   claims and are replaced. `X-Forwarded-For` names the peer, `X-Forwarded-Host`
   the host it asked for, and `X-Forwarded-Proto` plain `http`. Every other
   header whose name starts with `X-Forwarded-` is removed, and so are
-  `Forwarded` and `X-Real-IP`.
+  `Forwarded` and `X-Real-IP`. A name spelt with `_` for `-`, such as
+  `X_Forwarded_Ssl`, counts as the same name.
 - `X-Request-ID`, which carries the fresh id.
 - `X-Client-ASN` and `X-Client-Country`, which carry what `waf` answered. A
   header it did not give is left out. Any of the two the visitor sent is