epee: optionally restrict HTTP service to a configurable user agent

This is intended to catch traffic coming from a web browser,
so we avoid issues with a web page sending a transfer RPC to
the wallet. Requiring a particular user agent can act as a
simple password scheme, while we wait for 0MQ and proper
authentication to be merged.

contrib/epee/include/net/http_protocol_handler.h

@@ -49,6 +49,7 @@ namespace net_utils
 		struct http_server_config
 		{
 			std::string m_folder;
+			std::string m_required_user_agent;
 			critical_section m_lock;
 		};