Change VG_(nuke_all_threads) to disassociate the the stacks of the threads

Change VG_(nuke_all_threads) to disassociate the the stacks of the threads
being killed from the threads rather than marking them as inaccessible.

This should fix the problem with the environment (and other data from the
stacks of other threads) causing warnings after a fork. I believe that
VG_(nuke_all_threads) is only called in places where this is the behaviour
that we want or where it doesn't matter because we're about to exit anyway.

MERGE TO STABLE

git-svn-id: svn://svn.valgrind.org/valgrind/branches/VALGRIND_2_2_0_BRANCH@3033