Add support for `memfd` (on Linux) for storing scripts

[cipriancraciun]

At the moment scripts are passed to the interpreter as a /dev/fd/{N} path, with the descriptor being a pipe if the script is less than 1MiB, or a temporary file if it is larger.

On Linux there is a third alternative (perhaps better than both) by using a memfd, as per:

• https://lwn.net/Articles/593918/
• http://man7.org/linux/man-pages/man2/memfd_create.2.html