atrf-txrx can now run a shell command while emitting a test wave
atrf-txrx can now run a shell command while emitting a test wave

- atrf-txrx.c (test_mode, usage, main): optionally run a shell command while
  in test mode
- atrf-txrx.c (usage): explain all parameters

cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan