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.
Use the Windows API CryptProtectMemory() to encrypt the data that we want to
secure, and use CryptUnprotectMemory() to de-crypt the secured data that we
want to access, since mmap() and mlock() are not available on Windows.
