Small changes in callgrind_annotate and callgrind manual

Small changes in callgrind_annotate and callgrind manual
* callgrind_annotate: ignore the lines giving the long names of the
  events: such lines are used by kcachegrind for the GUI, so are better
  accepted (and ignored) by callgrind_annotate
* Document in callgrind_annotate manual that thresholds can be given
  per event kind.
* Avoid a division by zero in callgrind_annotate, for 'special data'
  such as produced by (some) xtrees.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16125