Use preauth options when changing password

If we try to change the password in rb5_get_init_creds_password, we
must use all application-specified gic options which affect
preauthentication when getting the kadmin/changepw ticket.  Create a
helper function make_chpw_options which copies the application's
options, unsets the options we don't want, and sets options
appropriate for a temporary ticket.

ticket: 7868