Faster and cleaner exit from `hackrf_transfer`

[mossmann]

hackrf_transfer could use some improvements similar to those made in hackrf_sweep over the past year.

• Check for completion more often than once per second (c47f2b1, 5361b3a).
• Remove extra calls to hackrf_stop_rx() and hackrf_stop_rx() when hackrf_close() is about to be called anyway (4c46fc7).
• Maybe jump to exit earlier, perhaps when the -n option is used (ab4498f).