drd: Add command-line option --verify-conflict-set

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14242 a5019735-40e9-0310-863c-91ae7b9d1cf9
drd/tests/threaded-fork-vcs: Add

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14243 a5019735-40e9-0310-863c-91ae7b9d1cf9
drd: Clear conflict set after fork (#338115)

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14244 a5019735-40e9-0310-863c-91ae7b9d1cf9