Suppress the known warnings generated by filesort().

The real fix belongs to worklog 1509:
Pack values of non-sorted fields in the sort buffer
(which is basically the same issue, but in an optimization context:
 We are writing the entire sort buffer to disk, 
 including un-used space for varchar columns.)



mysql-test/valgrind.supp:
  Add new Memcheck suppressions for filesort.
sql/filesort.cc:
  Remove the ifdef HAVE_purify/bzero code, use valgrind suppressions instead.