krb5_get_init_creds_opt_set_change_password_prompt() was being ignored by
krb5_init_creds_step() which broke pam_krb5 tests. MIT doesn't handle password
expiration within krb5_init_creds_step(), instead deferring to higher level
functions such as krb5_get_init_creds_password(). However, Heimdal kinit uses
krb5_init_creds_step() directly and thus requires this behaviour to be
implemented to pass its own tests.
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