MINOR: lua-thread: Replace "struct hlua_function" allocation by dedicated function

MINOR: lua-thread: Replace "struct hlua_function" allocation by dedicated function

The goal is to allow execution of one main lua state per thread.

This function will initialize the struct with other things than 0.
With this function helper, the initialization is centralized and
it prevents mistakes. This patch also keeps a reference to each
declared function in a list. It will be useful in next patches to
control consistency of declared references.