Add support for configuring mod_auth_kerb.

Two new configuration variables are recognized:
* CONF_BROKER_KRB_SERVICE_NAME for the KrbServiceName
* CONF_BROKER_KRB_AUTH_REALMS for the KrbAuthRealms
If both are specified, mod_auth_kerb and AuthType Kerberos are
configured with these values, instead of the default AuthType
Basic.