Display values in info box only if value is interesting

Type duration_t changed from uint to int.

Default value of '-1' introduced for some of the values in struct sample:
NDL used -1 as default.
Bearing uses -1 as default (no bearing set).

Display pXX, EAD, END, density, MOD only if values are larger than 0.

In profile don't display data from two first and two last plot_data
entries in info box.

Signed-off-by: Stefan Fuchs <sfuchs@gmx.de>

core/units.h

@@ -42,8 +42,11 @@ extern "C" {
  * We also strive to make '0' a meaningless number saying "not
  * initialized", since many values are things that may not have
  * been reported (eg cylinder pressure or temperature from dive
- * computers that don't support them). But sometimes -1 is an even
- * more explicit way of saying "not there".
+ * computers that don't support them). But for some of the values
+ * 0 doesn't works as a flag for not initialized. Examples are
+ * compass bearing (bearing_t) or NDL (duration_t).
+ * Therefore some types have a default value which is -1 and has to
+ * be set at certain points in the code.
  *
  * Thus "millibar" for pressure, for example, or "millikelvin" for
  * temperatures. Doing temperatures in celsius or fahrenheit would
@@ -70,7 +73,7 @@ typedef int64_t timestamp_t;
 
 typedef struct
 {
-	uint32_t seconds; // durations up to 68 yrs
+	int32_t seconds; // durations up to 34 yrs
 } duration_t;
 
 typedef struct