Code review comment for ~dbungert/curtin:cryptoswap-dmc-type

Can you expand the commit message a whole bunch please? This is about the case where /dev/random is used as a keyfile, in which case the dm type of the device would be created as luks in the live system but then be plain in the installed system and changing to a world where it will be plain in both?

Code changes look good but I have a worry about temporary file handling.