Make possible multiple C-R-like files

Change the function that creates the path for challenge-response
storate to take a "suffix" argument, so we can have multiple files.
Isolate the part of the challenge-response forming function that
fills in the expected response into a callback, so we now can store
other things in the "expected response" field.
Enable saved encrypted authtoken (password) p.1

Implement a second 'action' of ykpamcfg to create a file containing
challenge and a secret string encrypted with the response (by OTP).
Future update in the PAM module will inject this string as
PAM_AUTHTOKEN, so it could be used by subsequent PAM modules.