MINOR: hlua: support for optional arguments to core.register_task()

MINOR: hlua: support for optional arguments to core.register_task()

core.register_task(function) may now take up to 4 additional arguments
that will be passed as-is to the task function.
This could be convenient to spawn sub-tasks from existing functions
supporting core.register_task() without the need to use global
variables to pass some context to the newly created task function.

The new prototype is:

  core.register_task(function[, arg1[, arg2[, ...[, arg4]]]])

Implementation remains backward-compatible with existing scripts.