atrf-path: new option -T to sweep only one offset

atrf-path -T +0.5 -g ... 10 10 time:

before (both offsets)	0.65 s
after (one offset)	0.32-0.33 s	(50%)

- atrf-path.c (usage, main): new option -T to specify which offset to
  sweep (default: sweep both)
- atrf-path.c (do_half_sweep): only sample points with the desired offset
- atrf-path.c (print_sweep): only prints points we've sweeped
- gui.c (segment, draw): inverted flag logic from "have_last" to "first"
- gui.c (draw): only plot points we've sweeped
- gui.c (gui): pass the offset selection to "draw"

tools/atrf-path/sweep.h

@@ -14,6 +14,8 @@
 #ifndef	SWEEP_H
 #define	SWEEP_H
 
+#include <stdint.h>
+
 #include "atrf.h"
 
 
@@ -23,6 +25,7 @@ struct sweep {
 	int trim_tx;
 	int trim_rx;
 	int power;
+	uint8_t cont_tx;
 	int samples;
 };