This CR covers a list of improvements to G1's debugging output, which I found very helpful during my last round of G1 debugging. Here's a summary:

1) The ConcurrentMark::print_reachable() has been improved with the option to print either only the live or all objects in the heap and has been changed so that the output is more concise.

2) Upon a marking verification failure (when a live object points to a dead one), the output shows more information for the objects involved including their class names as well as their containing regions.

3) When +G1PrintHeapRegions is set, we were not printing the first region to be designated as to-space (the one we were remembering from the previous collection)

4) When +G1PrintHeapRegions is set, the output for regions in the CSet has been changed to make the distinction between young / not young a bit obvious.

5) When doing a region dump (with G1CollectedHeap::print_on_extended()), now the line of each region contains the NTAMS / PTAMS information.

See the Public Comments for more details.