win7 drmemory follow_children not working with -syscall_sentinels

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on February 03, 2011 14:22:59_

after fixing DRi#349:

follow children for drmem works on xp, and on win7 -leaks_only works and
other clients work, but on win7 regular drmem the child dies early:
kernelbase.dll init fails.

turns out it's from unknown syscall handling.
first, I fixed two bugs: wasn't restoring quite properly, and wasn't checking for
overlap, so a later param that overlaps would have the sentinel recorded as
the original value!

but after the fixes, still same behavior: the conclusion is that some of
this shadow-undefined memory is actually an input value to the syscall and
it really is defined. currently working on tracking that down. but the
risks of the sentinel approach are being hit here.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=287_