atrf-txrx: cleaned up the usage output
atrf-txrx: cleaned up the usage output

- atrf-txrx.c (usage): split options and arguments for different modi into
  separate sections
- atrf-txrx.c (usage): moved details for common options to a separate
  section
- atrf-txrx.c (usage): added -C mhz to synopsis

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