Client address is the right-most untrusted X-Forwarded-For entry (closes #144)

From a peer in TRUSTED_PROXIES, clientIP took the left-most
X-Forwarded-For entry, which the client writes itself behind a proxy
that appends to the header, so a client could choose the address the
rate limit counts. It now walks the entries from the right past trusted
proxies and takes the first that is not one. An entry with a port is
read as its address; with only trusted entries, or one that is not an
address at all, it uses the peer. Every X-Forwarded-For header line is
read, and X-Real-IP only when there is no X-Forwarded-For entry.

Model: opus-5-5

README.md

@@ -212,8 +212,15 @@ replaces the default; set and empty, the server trusts no proxy. Loopback is not
 in the default, so a reverse proxy on the same host has to be named, as in
 `TRUSTED_PROXIES=127.0.0.1/32`. A request whose direct peer is outside this set
 has its forwarded headers ignored, and the direct peer is logged and
-rate-limited instead. The container image does not use this default; see
-[Deployment](#deployment).
+rate-limited instead. From a direct peer inside the set, the client address is
+the right-most `X-Forwarded-For` entry that is not in the set: each proxy
+appends the address it took the request from, so the entries further left were
+written by the client and are not used. An entry written as an address with a
+port, such as `198.51.100.7:5000` or `[2001:db8::1]:443`, is read as that
+address. If the search from the right finds only addresses in the set, or meets
+an entry that is not an address at all, such as `unknown`, the direct peer is
+used. `X-Real-IP` is read only when `X-Forwarded-For` is missing or empty. The
+container image does not use this default; see [Deployment](#deployment).
 
 A variable set to a value the server cannot use, such as `PORT=abc`,
 `DEBUG=maybe` or a `BIND_ADDRESS` that is not an IP address, stops it from