debugstack([thread,] [start [, count1 [, count2]]]) returns the stack
of a coroutine when one is passed first, with start defaulting to 0
(the coroutine's innermost level), as 4.3.4's Script_debugstack does.
1.12.1's debugstack (0x703760) has no thread argument. A thread fails
its lua_isnumber and lua_isstring checks, so it returns the thread
itself, and msg .. debugstack(co) raises "attempt to concatenate a
thread value" in the coroutine error paths of WeakAuras, Questie's
ThreadLib, MRT and Zygor.
The engine function still builds the text, so its line format and its
12/10-level head and tail windows are unchanged, which keeps addons
that parse debugstack() for their folder working. It reads levels
through six calls on the state it runs on: lua_getstack (0x6FBAA0) at
0x70383F, 0x70386E, 0x703896, 0x7038AD and 0x703998, and
lua_getinfo (0x6FBC70) at 0x7038CD. Those calls are redirected to
read from the state the new global names for the call: the thread, or
the calling state. The engine function has no callers of its own, so
the global is its only way in and sets that state for every call; an
error part way cannot leave a thread behind for the next call.
The global removes the thread argument, inserts 0 as start when none
is given, and calls the engine function directly in its own frame, so
levels without a thread count exactly as before.
