Change user_state 'challenge' field from string to uchar, which

	Change user_state 'challenge' field from string to uchar, which
	makes more sense for crypto and other manipulation.  Unfortunately,
	it is a huge change for such a small gain (basically avoiding
	sprintf() and sscanf() conversions.)

	Notable changes:
	- otp.h: add 'clen' field to user_state; update it wherever the
	  'challenge' field is set
	- otp_cardops.h: add printchallenge() method, OTP_CF_C8 and OTP_CF_C4
	  Card Feature macros
	- cardops keystring2keyblock() method and otp_keystring2keyblock()
	  now return keylen (instead of 0) on success
	- otp_keyblock2keystring() now takes a length arg and returns char *
	  for easy printing
	- remove [unused] 'card_info' and 'log_prefix' args to cardops
	  updatecsd() method
	- otp_gen_challenge() noted as cryptocard-specific with quirky behavior
	- otp_challenge_transform() explicitly returns new challenge length
        - cryptocard.c:cryptocard_challenge(): Remove hardcoded challenge
	  length of 8.  Yes, *sync* challenge length must be 8 but challenge
	  might be a different size due to resync with an async challenge.
	  This worked before but became a bug when we changed the user_state
	  'challenge' field to be the previous (instead of the next) challenge.