MINOR: lua: add REGISTER_HLUA_STATE_INIT() to register state init callbacks

MINOR: lua: add REGISTER_HLUA_STATE_INIT() to register state init callbacks

Add a registration mechanism so that modules outside of hlua.c can hook
into each lua_State creation. Modules call hap_register_hlua_state_init()
(or the REGISTER_HLUA_STATE_INIT() macro) with a callback of the form:

  int my_init(lua_State *L, char **errmsg);

The callback returns an ERR_* code. ERR_ALERT and ERR_WARN trigger
ha_alert()/ha_warning() respectively; any other non-zero errmsg is
emitted via ha_notice(). ERR_FATAL or ERR_ABORT cause exit(1).
Registered entries are freed in hlua_deinit().