Minimal support for updating history key

Minimal support for updating history key

Add minimal support for re-randomizing the history key:

* cpw -randkey kadmin/history now works, but creates only one key.
* cpw -randkey -keepold kadmin/history still fails.
* libkadm5 no longer caches the history key.  Performance impact
  is minimal since password changes are not common.
* randkey no longer checks the newly randomized key against old keys,
  and the disabled code to do so in setkey/setv4key is gone, so now
  only kadm5_chpass_principal_3 accesses the password history.

ticket: 6660
target_version: 1.8
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@23716 dc483132-0cff-0310-8789-dd5450dbe970