middleware: take the client address from the right of X-Forwarded-For (closes #181)

realIP took the first X-Forwarded-For entry, which the client itself
can write, so behind a proxy that appends to the header a client chose
the address dnswatcher logs and the /metrics rate limit counts. It now
walks the entries from the right past trusted proxies, using the
existing trusted-proxy check, and takes the first that is not one; the
leftmost when all are. All X-Forwarded-For header lines are read as one
list, since a proxy may add its own line instead of appending to the
client's. An empty entry where the client address belongs falls back
to the peer address, as an empty first entry did before. X-Real-IP is
unchanged.

Model: opus-5-5

README.md

@@ -347,9 +347,9 @@ minute, failed logins included; beyond that it answers `429 Too Many Requests`
 without checking the password. A Prometheus server scraping every 15 seconds
 sends 4 a minute. IPv6 addresses in one /64 count as one client. When the
 request comes from a private or loopback address, such as a reverse proxy's,
-the client address is taken from the `X-Real-IP` or `X-Forwarded-For` header
-the proxy sets; a proxy that sets neither makes all its clients share one
-allowance.
+the client address is taken from the `X-Real-IP` header the proxy sets, or else
+from `X-Forwarded-For`, as the last address in it that is not private or
+loopback. A proxy that sets neither makes all its clients share one allowance.
 
 **`DNSWATCHER_DNS_INTERVAL` and `DNSWATCHER_TLS_INTERVAL`** take a positive
 duration: a number followed by a unit such as `s`, `m` or `h`, for example