cryptsetup: drop "headless" bool, make it a flag in AskPasswordFlags instead

cryptsetup: drop "headless" bool, make it a flag in AskPasswordFlags instead

This bool controls whether we should interactively ask for a password,
which is pretty much what the ask_password-api.c APIs are about. Hence,
just make the bool a flag in AskPasswordFlags enum, and use it
everywhere.

This still catches the flag early in upper levels of the codebase,
exactly as before, but if the flag is still present in the lower layers
it's also handled there and results in ENOEXEC if seen.

This is mostly an excercise in simplifying our ridiculously long
function call parameter lists a bit.