Define MD5_K5BETA_COMPAT and MD5_K5BETA_COMP_DEF so that we continue

Define MD5_K5BETA_COMPAT and MD5_K5BETA_COMP_DEF so that we continue
doing things the wrong (broken) way.  All hail backwards
compatibility....  The code now generates the old checksum, but it
will verify both the old and the correct checksum formats.

Also fixed two bugs in the "correct" MD5_CRYPTO implementation; use a
zero initialization vector, and calculate the confounder at the
beginning of the message, not at the end.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@8044 dc483132-0cff-0310-8789-dd5450dbe970