Don't dump cache pages in cores

[PerilousApricot]

Playing around with automagically collecting cores, if the process has a lot of cache, the dumps can be very large. We should instead do a madvise(MADV_DONTDUMP) on any cache allocations to keep the core size manageable. One hitch is that Centos6' kernel is too old to support it, so we'll need to wrap it in #if defines where CMake does a configuration-time test to see if MADV_DONTDUMP is declared