DRD changes (Bart Van Assche)

DRD changes (Bart Van Assche)

* Add docs: exp-drd/docs/README.txt
* Added one drd suppression pattern, and cleaned up the suppression file.
* All regression tests now pass on x86_64 and i386, including sigalrm.
* Updated TODO.txt file.
* pth_create_chain test is now started with 100 threads instead of 10
  -- 10 was not enough.
* DRD no longer exits on PPC32 and PPC64 but just prints a warning
  message before it starts.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@7275