krb5_child: advertise authentication methods

During the `preauthentication` phase krb5_child checks for the available
authentication methods for the given user, advertises them and the
process is kept alive. Once the state is change to `authentication` the
same krb5_child process processes the credentials and proceeds with the
authentication itself.

Signed-off-by: Iker Pedrosa <ipedrosa@redhat.com>

Author: ikerexxe