The function isn't used anymore since each call place performs its
own loop. Let's get rid of it.

The thread dump mechanism that is used by "show threads" and by the
panic dump is overly complicated due to an initial misdesign. It
firsts wakes all threads, then serializes their dumps, then releases
them, while taking extreme care not to face colliding dumps. In fact
this is not what we need and it reached a limit where big machines
cannot dump all their threads anymore due to buffer size limitations.

What is needed instead is to be able to dump *one* thread, and to let
the requester iterate on all threads.

That's what this patch does. It adds the thread_dump_buffer to the
struct thread_ctx so that the requester offers the buffer to the
thread that is about to be dumped. This buffer also serves as a lock.
A thread at rest has a NULL, a valid pointer indicates the thread is
using it, and 0x1 (NULL+1) is used by the dumped thread to tell the
requester it's done. This makes sure that a given thread is dumped
once at a time. In addition to this, the calling thread decides
whether it accesses the thread by itself or via the debug signal
handler, in order to get a backtrace. This is much saner because the
calling thread is free to do whatever it wants with the buffer after
each thread is dumped, and there is no dependency between threads,
once they've dumped, they're free to continue (and possibly to dump
for another requester if needed). Finally, when the THREAD_DUMP
feature is disabled and the debug signal is not used, the requester
accesses the thread by itself like before.

For now we still have the buffer size limitation but it will be
addressed in future patches.

This flag is not needed anymore as we're already marking the waiting
threads as harmless, thus the thread's bit is already covered by this
information. The variable was unexported.

"f(void)" is the correct and preferred form for a function taking no
argument, while some places use the older "f()". These were reported
by clang's -Wmissing-prototypes, for example:

  src/cpuset.c:111:5: warning: no previous prototype for function 'ha_cpuset_size' [-Wmissing-prototypes]
  int ha_cpuset_size()
  include/haproxy/cpuset.h:42:5: note: this declaration is not a prototype; add 'void' to make it a prototype for a zero-parameter function
  int ha_cpuset_size();
      ^
                     void

This aggregate patch fixes this for the following functions:

   ha_backtrace_to_stderr(), ha_cpuset_size(), ha_panic(), ha_random64(),
   ha_thread_dump_all_to_trash(), get_exec_path(), check_config_validity(),
   mworker_child_nb(), mworker_cli_proxy_(create|stop)(),
   mworker_cleantasks(), mworker_cleanlisteners(), mworker_ext_launch_all(),
   mworker_reload(), mworker_(env|proc_list)_to_(proc_list|env)(),
   mworker_(un|)block_signals(), proxy_adjust_all_maxconn(),
   proxy_destroy_all_defaults(), get_tainted(),
   pool_total_(allocated|used)(), thread_isolate(_full|)(),
   thread(_sync|)_release(), thread_harmless_till_end(),
   thread_cpu_mask_forced(), dequeue_all_listeners(), next_timer_expiry(),
   wake_expired_tasks(), process_runnable_tasks(), init_acl(),
   init_buffer(), (de|)init_log_buffers(), (de|)init_pollers(),
   fork_poller(), pool_destroy_all(), pool_evict_from_local_caches(),
   pool_total_failures(), dump_pools_to_trash(), cfg_run_diagnostics(),
   tv_init_(process|thread)_date(), __signal_process_queue(),
   deinit_signals(), haproxy_unblock_signals()

The dump state is now passed to the function so that the caller can adjust
the behavior. A new series of 4 values allow to stop *after* dumping main
instead of before it or any of the usual loops. This allows to also report
BUG_ON() that could happen very high in the call graph (e.g. startup, or
the scheduler itself) while still understanding what the call path was.

This function calls the ha_dump_backtrace() function with a locally
allocated buffer and sends the output slightly indented to fd #2. It's
meant to be used as an emergency backtrace dump.

The backtrace dumping code was located into the thread dump function
but it looks particularly convenient to be able to call it to produce
a dump in other situations, so let's move it to its own function and
make sure it's called last in the function so that we can benefit from
tail merging to save one entry.

This one was introduced 5 years ago for debugging and never really used.
It is the one which used to cause circular dependencies issues. Let's drop
it instead of starting to split the debug include in two.

The debug file is cleaner now and does not depend on much anymore.