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.

src/daemon/command_server.h

@@ -54,6 +54,7 @@ public:
   t_command_server(
       uint32_t ip
     , uint16_t port
+    , const std::string &user_agent
     , bool is_rpc = true
     , cryptonote::core_rpc_server* rpc_server = NULL
     );