Keep a redirect on the unix socket in ClientImpl::redirect (#2658)

A client using set_address_family(AF_UNIX) took the host of an absolute Location as a socket path, so a redirect to http://localhost/next failed and one to http://other.sock/next went to ./other.sock. Every hop now stays on the client's socket, as with curl's --unix-socket.

The same host is still decided from the Location, as curl does: only localhost (the Host this client sends) counts as this client, so a redirect to any other host goes through the cross-host client and drops Authorization and Cookie. That client reaches the original socket through addr_map_, and a host the caller mapped with set_direct_hostname_addr_map keeps its mapping. An https Location runs TLS over the socket with the Location's host.

The mapped-target lookup shared with make_connect_request_target is factored into detail::addr_map_target.

README.md

@@ -1716,6 +1716,8 @@ cli.set_address_family(AF_UNIX);
 
 This library automatically sets the Host header to "localhost" for Unix socket connections, similar to curl's behavior.
 
+With `set_follow_location(true)`, a redirect is followed over the same socket, whatever host the `Location` names, as curl's `--unix-socket` does. A host mapped with `set_direct_hostname_addr_map` goes to its mapped socket instead. Only a `Location` naming `localhost` counts as the same host; any other host is treated as cross-host, so `Authorization` and `Cookie` are not sent to it.
+
 
 URI Encoding/Decoding Utilities
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