MINOR: tasks: add DEBUG_TASK to report caller info in a task

MINOR: tasks: add DEBUG_TASK to report caller info in a task

The idea is to know who woke a task up, by recording the last two
callers in a rotating mode. For now it's trivial with task_wakeup()
but tasklet_wakeup_on() will require quite some more changes.

This typically gives this from the debugger:

  (gdb) p t->debug
  $2 = {
    caller_file = {0x0, 0x8c0d80 "src/task.c"},
    caller_line = {0, 260},
    caller_idx = 1
  }

or this:

  (gdb) p t->debug
  $6 = {
    caller_file = {0x7fffe40329e0 "", 0x885feb "src/stream.c"},
    caller_line = {284, 284},
    caller_idx = 1
  }

But it also provides a trivial macro allowing to simply place a call in
a task/tasklet handler that needs to be observed:

   DEBUG_TASK_PRINT_CALLER(t);

Then starting haproxy this way would trivially yield such info:

  $ ./haproxy -db -f test.cfg | sort | uniq -c | sort -nr
   199992 h1_io_cb woken up from src/sock.c:797
    51764 h1_io_cb woken up from src/mux_h1.c:3634
       65 h1_io_cb woken up from src/connection.c:169
       45 h1_io_cb woken up from src/sock.c:777