message: backend: add connection URL as arg to  backend_bin
  This allows backend_bin to accept the connection URL as an argument, as
  well as -h or --help to show the usage.
diff: #include "backend_api.h"
  +#include <iostream>
  +#include <string>
  +
  +static void usage();
  +
  +static auto constexpr default_connection = "udp://:14540";
  int main(int argc, char **argv)
  {
  - runBackend("udp://14540", nullptr, nullptr);
  + if (argc > 2) {
  + usage();
  + return 1;
  + }
  +
  + if (argc == 2) {
  + const std::string positional_arg = argv[1];
  + if (positional_arg.compare("-h") == 0 || positional_arg.compare("--help") == 0) {
  + usage();
  + return 0;
  + }
  + runBackend(positional_arg.c_str(), nullptr, nullptr);
  + }
  +
  + runBackend(default_connection, nullptr, nullptr);
  +}
  +
  +void usage()
  +{
  + std::cout << "Usage: backend_bin [-h | --help]" << std::endl
  + << " backend_bin [Connection URL]" << std::endl
  + << std::endl
  + << "Connection URL format should be:" << std::endl
  + << " Serial: serial:///path/to/serial/dev[:baudrate]" << std::endl
  + << " UDP: udp://[bind_host][:bind_port]" << std::endl
  + << " TCP: tcp://[server_host][:server_port]" << std::endl
  + << std::endl
  + << "For example to connect to SITL use: udp://:14540" << std::endl
  + << std::endl
  + << "Options:" << std::endl
  + << " -h | --help : show this help" << std::endl;
  }
  