MINOR: lua: file dedicated to unsafe functions

MINOR: lua: file dedicated to unsafe functions

When Lua executes functions from its API, these can throws an error.
These function must be executed in a special environment which catch
these error, otherwise a critical error (like segfault) can raise.

This patch add a c file called "hlua_fcn.c" which collect all the
Lua/c function needing safe environment for its execution.