MINOR: hlua: add a new hlua_show_current_location() function

MINOR: hlua: add a new hlua_show_current_location() function

This function may be used to try to show where some Lua code is currently
being executed. It tries hard to detect the initialization phase, both for
the global and the per-thread states, and for runtime states. This intends
to be used by error handlers to provide the users with indications about
what Lua code was being executed when the error triggered.