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/rpc_command_executor.cpp

@@ -92,6 +92,7 @@ namespace {
 t_rpc_command_executor::t_rpc_command_executor(
     uint32_t ip
   , uint16_t port
+  , const std::string &user_agent
   , bool is_rpc
   , cryptonote::core_rpc_server* rpc_server
   )