UI should lazy init so that init and fini are only called if one

UI should lazy init so that init and fini are only called if one
of the UI calls is called.  The problem is that when you call
krb5_get_init_creds_* you don't know if it will call the prompter
or not.  (It won't if the password is saved in the keychain or
if pkinit succeeds.)

ticket: 6055

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@20780 dc483132-0cff-0310-8789-dd5450dbe970