Make sure Agez secrets aren't leaked to storage

[c0c0n3]

At the moment the Agez module uses tmpfs as a storage backend. While this is a RAM filesystem, some of its contents could be swapped out to disk if the system has a swap partition or file. Ideally we should use ramfs like Agenix does, but this is more work which is why we opted for tmpfs initially.

See also:

• https://unix.stackexchange.com/questions/50482

[c0c0n3]

Before going ahead with this, consider #22.