mark other thread stacks as unaddr in fork child

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on September 14, 2011 13:16:53_

after a fork, it seems reasonable to report an access to a (now-gone)
thread's stack as an error. this requires recording the other threads
pre-fork and in the child marking each of their stacks as completely
unaddressable.

this came up over discussion re: https://bugs.kde.org/show_bug.cgi?id=282018

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=592_