MEDIUM: debug: make the thread dump code show Lua backtraces

MEDIUM: debug: make the thread dump code show Lua backtraces

When we dump a thread's state (show thread, panic) we don't know if
anything is happening in Lua, which can be problematic especially when
calling external functions. With this patch, the thread dump code can
now detect if we're running in a global Lua task (hlua_process_task),
or in a TCP or HTTP Lua service (task_run_applet and applet.fct ==
hlua_applet_tcp_fct or http_applet_http_fct), or a fetch/converter
from an analyser (s->hlua != NULL). In such situations, it's able to
append a formatted Lua backtrace of the Lua execution path with
function names, file names and line numbers.

Note that a shorter alternative could be to call "luaL_where(hlua->T,0)"
which only prints the current location, but it's not necessarily sufficient
for complex code.