BUG/MEDIUM: debug/lua: Don't dump the lua stack if not dumpable

BUG/MEDIUM: debug/lua: Don't dump the lua stack if not dumpable

When we try to dump the stack of a lua context, if it is not dumpable,
nothing is performed and a message is emitted instead. This happens when a
lua execution was interrupted inside a non-reentrant part.

This patch depends on following commit :

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

Thanks to this patch, we avoid a possible deadllock if the lua is
interrupted by the watchdog in the lua memory allocator, because realloc()
is not async-signal-safe.

Both patches must be backported as far as 2.0.

(cherry picked from commit 83926a04febe94f332c6f45c98773286678346d3)
Signed-off-by: Willy Tarreau <w@1wt.eu>