Make it easier to test SAM-2 client code

Make it easier to test SAM-2 client code

Add a method to the securid_sam2 plugin, built with alternate
compile-time flags, which supplies a plain-text challenge to the
client to be used as the OTP value.  This lets us manually exercise
the SAM-2 client code and a little bit of the KDC code.

securid_make_sam_challenge_2_and_cksum is moved into the method-
independent code and renamed.  get_securid_edata_2 has its sc2b
parameter removed as it was not used by the caller.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@25832 dc483132-0cff-0310-8789-dd5450dbe970