Handle dates and a "calculated plural" in translations

This looks like a massive commit mainly because of the line number changes
in the .po files. That sadly hides what really happened here:

- the places where we manually build dates have now been localized
- the one place where we did the English "calculated plural" has been
  modified so that it now can be correctly translated (in English this
  just adds an 's' to the noun if the number is != 1 - in other languages
  this tends to be much more complicated)

I then updated the two German translations to take advantage of the new
constructs. And while I was at it, I changed the translation Trip->Gruppe
to Trip->Reise as that seemed much more appropriate.
I also fixed another error in the German translation where I translated
"dive time" as "Startzeit" - but in the context it was "Dauer".

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

info.c

@@ -103,7 +103,8 @@ static int divename(char *buf, size_t size, struct dive *dive)
 	struct tm tm;
 
 	utc_mkdate(dive->when, &tm);
-	return snprintf(buf, size, _("Dive #%d - %s %02d/%02d/%04d at %d:%02d"),
+	/*++GETTEXT 80 char buffer: dive nr, weekday, month, day, year, hour, min */
+	return snprintf(buf, size, _("Dive #%1$d - %2$s %3$02d/%4$02d/%5$04d at %6$d:%7$02d"),
 		dive->number,
 		weekday(tm.tm_wday),
 		tm.tm_mon+1, tm.tm_mday,