Add message hash support to the replay interface, using extension

Add message hash support to the replay interface, using extension
records (with an empty client string) to retain compatibility with old
code.  For rd_req, the ciphertext of the authenticator (with no ASN.1
wrapping) is hashed; for other uses of the replay cache, no message
hash is used at this time.

This commit adds a command-line tool for testing the replay cache but
does not add any automated tests.

ticket: 1201

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@21723 dc483132-0cff-0310-8789-dd5450dbe970