egg: Request that secure memory not be dumped to disk

Linux 3.4 added support for the MADV_DONTDUMP option to madvise(), which requests that the covered memory not be included in coredumps. It makes sense to use this to prevent cases where application crashes could result in secrets being persisted to disk or included in dumps that are uploaded to remote servers for analysis. I've avoided making this fatal since there's a chance this code could be built on systems that have MADV_DONTDUMP but run on systems that don't.