Fix ask_for_password bug breaking bash without plymouth

Fix ask_for_password bug breaking bash without plymouth

If crypt-lib.sh is sourced from any #!/bin/sh script, a POSIX shell
quirk is in effect that causes variable assignments to "special
builtins" (such as "shift") to leak to their context. So the buggy
code works even despite the missing semicolons.

But if it is sourced by "bash acting under its own name", i.e. from
any #!/bin/bash script, the quirk is disabled, tty_cmd/tty_prompt are
undefined, and ask_for_password doesn't do anything if plymouth is not
present.