atrf-path: added pass/fail indication (in the GUI, accept the result with P/F)

- atrf-path.c (do_sweeps): return a pass/fail/undecided value
- gui.h (gui), gui.c (gui): return a pass/fail/undecided value
- gui.c (gui): return pass/fail when P or F is pressed; exit
  unconditionally if Q is pressed
- atrf-path.c (main): according to the decision of "gui" or "do_sweeps",
  print "#PASS", "#FAIL", or nothing

tools/atrf-path/gui.h

@@ -18,10 +18,18 @@
 #include "sweep.h"
 
 
+/*
+ * gui returns one of the following values:
+ *
+ * -1	fail
+ *  0	no decision taken
+ *  1	pass
+ */
+
 #ifdef HAVE_GFX
-void gui(const struct sweep *sweep, int sweeps);
+int gui(const struct sweep *sweep, int sweeps);
 #else
-#define gui(sweep, sweeps) abort()
+#define gui(sweep, sweeps) ({ abort(); 0; })
 #endif
 
 #endif /* !GUI_H */