MEDIUM: lua: Use a per-thread counter to track some non-reentrant parts of lua

MEDIUM: lua: Use a per-thread counter to track some non-reentrant parts of lua

Some parts of the Lua are non-reentrant. We must be sure to carefully track
these parts to not dump the lua stack when it is interrupted inside such
parts. For now, we only identified the custom lua allocator. If the thread
is interrupted during the memory allocation, we must not try to print the
lua stack wich also allocate memory. Indeed, realloc() is not
async-signal-safe.

In this patch we introduce a thread-local counter. It is incremented before
entering in a non-reentrant part and decremented when exiting. It is only
performed in hlua_alloc() for now.

(cherry picked from commit a61789a1d62fd71c751189faf5371740dd375f33)
[wt: the code uses malloc/realloc/free depending on the inputs in 2.3,
 let's place the counter around these 3 calls]
Signed-off-by: Willy Tarreau <w@1wt.eu>