atrf-path/atrf-path.c: initialize the receiver only once, not for each sweep
atrf-path/atrf-path.c: initialize the receiver only once, not for each sweep

atrf-path -g ... 10 10 time:

before	2.08 s
after	1.92-1.93 s	(92-93%)

- atrf-path.c (set_channel, init_common): moved setting of the channel
  to separate function, for better granularity
- atrf-path.c (init_common, init_tx, init_rx, sample, do_sweep): the
  init_* functions no longer set the channel; use set_channel
- atrf-path.c (main): initialize the receiver only once

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