Is there interest in a feature gated memfd_secret backend?

[wucke13]

We are interested in having a memfd_secret based allocator, that uses memsec as fallback. So we figured it would make sense to ask, if this would be a functionality desired directly in memsec?

The basic ideas are:

• provide runtime function to check for memfd_secret support (this we already have, on non-linux systems that is compile time known false)
• if memfd_secret support is available, try memfd_secret + mmap to allocate
• if that fails, fall back to the default

All that of course feature gated, not active by default.

[quininer]

I think it's good, would you like to open a PR?