Set default valgrind options to "-v --show-reachable=yes" and make it possible to override that with --valgrind-options 


mysql-test/mysql-test-run.pl:
  Make --valgrind-all a synonym for --valgrind.
  Set "-v --show-reachable=yes" as our default valgrind options. 
  Make --valgrind-options override the default options
  Update usage.
  Turn on valgrinding also when user specified --valgrind-path or --valgrind-options without --valgrind