proxyserver: add support for UnifiedPush standard

Make the proxy able to support the UnifiedPush platform, so that
applications using the proxy server can use any UnifiedPush provider.

Documentation: https://unifiedpush.org/

How to test:
+ Install a UnifiedPush provider on a device (e.g. https://ntfy.sh)
+ Run dhtnode with a proxyserver and --unifiedpush https://ntfy.sh
+ Subscribe to a random topic on ntfy (e.g. "dhtproxytest")
+ Subscribe to a hash on the DHT:
    curl -X SUBSCRIBE http://my.dht.proxy:8000/key/foo \
         -d '{"key":"dhtproxytest", "platform":"unifiedpush"}'
+ Send data on the subscribed hash

Note: the topic should be completely random.

tools/dhtnode.cpp

@@ -43,7 +43,7 @@ void print_version() {
 }
 
 void print_usage() {
-    std::cout << "Usage: dhtnode [-v [-l logfile]] [-i] [-d] [-n network_id] [-p local_port] [-b bootstrap_host[:port]] [--proxyserver local_port] [--proxyserverssl local_port] [--bundleid bundleid]" << std::endl << std::endl;
+    std::cout << "Usage: dhtnode [-v [-l logfile]] [-i] [-d] [-n network_id] [-p local_port] [-b bootstrap_host[:port]] [--proxyserver local_port] [--proxyserverssl local_port] [--bundleid bundleid] [--openpush endpoint]" << std::endl << std::endl;
     print_info();
 }
 
@@ -552,6 +552,7 @@ main(int argc, char **argv)
             serverConfig.pushServer = params.pushserver;
             serverConfig.bundleId = params.bundle_id;
             serverConfig.address = params.proxy_address;
+            serverConfig.unifiedPushEndpoint = params.unifiedPushEndpoint;
             if (params.proxyserverssl and params.proxy_id.first and params.proxy_id.second){
                 serverConfig.identity = params.proxy_id;
                 serverConfig.port = params.proxyserverssl;