Match trusted proxies as IP addresses

set_trusted_proxies() compared addresses as strings, so a proxy listed
as 10.0.0.2 was not trusted when a server listening on both IPv4 and
IPv6 saw it as ::ffff:10.0.0.2, and two spellings of one IPv6 address
did not match either. The listed addresses are now put in one spelling
when they are set, and so are the connection peer and each
X-Forwarded-For hop before they are looked up; an IPv4-mapped IPv6
address becomes its IPv4 address. The README now documents the setter.

README.md

@@ -916,6 +916,14 @@ svr.set_keep_alive_max_count(2); // Default is 100, 0 for no limit
 svr.set_keep_alive_timeout(10);  // Default is 5
 ```
 
+### Client address behind a reverse proxy
+
+```cpp
+svr.set_trusted_proxies({"10.0.0.2"});
+```
+
+A request that comes straight from one of these addresses gets `req.remote_addr` from `X-Forwarded-For`: the rightmost address in it that is not a trusted proxy. A request from anywhere else keeps the address of its connection, whatever `X-Forwarded-For` says. Addresses match as IP addresses, so an IPv4 peer that a server listening on both IPv4 and IPv6 sees as `::ffff:10.0.0.2` matches `10.0.0.2`. Host names and CIDR ranges are not supported.
+
 ### Timeout
 
 ```c++