Massive cleanups to module instantiation.

The mod_instantiate() now takes void*instance, instead of
void**instance.

The module_t struct now has sizeof(rlm_foo_t), along with
CONF_PARSER*module_config.

The server core allocates instance = module->size
and runs CONF_PARSER (if available)

only if that passes does it call the mod_insantiate
routine