set default argv to help

A trivial patch to remove the confused message when people just
type "anbox" in their terminal.

  $ anbox
  Unknown command 'run'
  NAME:
      anbox - anbox
  ...

Author: zhsj

src/anbox/daemon.cpp

@@ -55,7 +55,7 @@ Daemon::Daemon()
 
 int Daemon::Run(const std::vector<std::string> &arguments) try {
   auto argv = arguments;
-  if (arguments.size() == 0) argv = {"run"};
+  if (arguments.size() == 0) argv = {"help"};
   return cmd.run({std::cin, std::cout, argv});
 } catch (std::exception &err) {
   ERROR("%s", err.what());