enable redzones in Debug CRT

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on March 12, 2012 23:20:52_

pattern mode is currently not detecting over/underflows w/ debug CRT b/c
there are no redzones

redzones are disabled because of issue #26 where debug operator delete goes and reads the heap header at a hardcoded negative offset.

originally they were disabled only for the _dbg routines themselves, but
there are asymmetries and abstraction violations seen in issue #500 that
resulted in having to disable redzones for all allocators in a module that contains _dbg routines

if we can handle redzones for issue #26 then the issue #500 mismatches shouldn't matter b/c those were all about redzone vs no redzone.

issue #26 alternative solutions to disabling redzones:
1) adjust on operator new:
note that this was the first thing I considered (from looking at my
notes) but at the time I was concerned about being able to find all
the operators via the drsyms interfaces of the time.

later I did write code having the operators as a first-class adjusting
layer in my initial issue #123 implementation but I reverted it b/c
I had trouble with placement new and placement delete.

to identify placement operators we need func params: we have that on
windows (DRi#605) but not on linux. and, there's no standard signature
for placement delete. we could make some assumptions, but are we sure
they'd be bulletproof? perhaps a 2nd param to operator delete might
be used for some other purpose in a custom usage.

2) hack: make location in our redzone where operator delete will find
header bit have the right bit set

it may well be that `#2` will end up being more robust (as well as more
efficient, since a full layer will require post-call interception of
operator new which we avoid today)

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=832_