Fix use of uninit heuristic set for monitor command
* initialise leak_check heuristic parameter in the leak_check monitor command
* show the default value (none heuristic) in the monitor help command
  (none value chosen as default as this gives a backward compatible
   behaviour).
* document the heuristic leak check parameter in memcheck manual
  monitor command section



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13631 a5019735-40e9-0310-863c-91ae7b9d1cf9