kpasswd: if a credential cache is present, use FAST

kpasswd: if a credential cache is present, use FAST

If a credentials cache is available, use it as an armor cache to enable FAST negotiation for kpasswd. This requires an attacker to attack both the user's long-term key for the old password as well as the ticket used for the armor cache in order to attack the password change. Depending on how the armor ticket is obtained, this may provide limited value. However, it provides users an easy option if they are concerned about their current password. Users can kinit with one principal to help protect changing the password of another principal.

* krb5_get_init_creds_opt_set_fast_ccache: new API to set fast ccache based on a krb5_ccache object rather than a resolvable string

* kpasswd: always open the current credential cache even if not needed
  for determining the principal. If the cache has tickets, use it as
  an armor cache.

* tests/dejagnu/krb-standalone/kadmin.exp: Arrange to test new code path

ticket: 6786

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24359 dc483132-0cff-0310-8789-dd5450dbe970