MINOR: lua-thread: Add the "thread" core variable

MINOR: lua-thread: Add the "thread" core variable

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

This commit introduces this variable in the core. Lua state initialized
by thread will have access to this variable, which reports the executing
thread. 0 indicates the shared thread. Programs which must be executed
only once can check for core.thread <= 1.