Small improvements to spectrum walk script.

- usrp/step: the default number of runs (10) can now be overridden on the
  command line
- usrp/step: usrp2_rx_cfile.py likes to busy loop from time to time. Add
  a CPU time limit to prevent this from getting out of hand.

usrp/step

@@ -2,7 +2,7 @@
 
 CH_FIRST=11
 CH_LAST=26
-RUNS=10
+RUNS=${1:-10}
 
 m10=0123456789
 m20=$m10$m10
@@ -23,7 +23,8 @@ while [ $run -lt $RUNS ]; do
         mhz $c | tr '\n' ' ' >>out
 	( ssh ben ./atspi-txrx -c $c -p 4 $MSG 1500; echo DONE 1>&2; ) &
 	sleep 3
-	usrp2_rx_cfile.py -d 4 -f `mhz $c`M -g 40 -N 100M tmp
+	( ulimit -t 60;
+	  usrp2_rx_cfile.py -d 4 -f `mhz $c`M -g 40 -N 100M tmp; )
 	sync
 	./p <tmp >>out
 	rm -f tmp