Made the error reporting behavior of DRD similar to that of other

Made the error reporting behavior of DRD similar to that of other
Valgrind tools: if the same kind of error has been detected a second
time with an identical call stack, count it as an error but do not print
it a second time. The total number of detected errors remains unchanged
but the number of error contexts is now reported correctly.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@10633