gtksecurememory: Request that secure memory not be dumped to disk

gtksecurememory: 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.