MINOR: debug: store important pointers in post_mortem

MINOR: debug: store important pointers in post_mortem

Dealing with a core and a stripped executable is a pain when it comes
to finding pools, proxies or thread contexts. Let's put a pointer to
these heads and arrays in the post_mortem struct for easier location.
Other critical lists like this could possibly benefit from being added
later.

Here we now have:
  - tgroup_info
  - thread_info
  - tgroup_ctx
  - thread_ctx
  - pools
  - proxies

Example:
  $ objdump -h haproxy|grep post
   34 _post_mortem  000014b0  0000000000cfd400  0000000000cfd400  008fc400  2**8

  (gdb) set $pm=(struct post_mortem*)0x0000000000cfd400

  (gdb) p $pm->tgroup_ctx[0]
  $8 = {
    threads_harmless = 254,
    threads_idle = 254,
    stopping_threads = 0,
    timers = {
      b = {0x0, 0x0}
    },
    niced_tasks = 0,
    __pad = 0xf5662c <ha_tgroup_ctx+44> "",
    __end = 0xf56640 <ha_tgroup_ctx+64> ""
  }

  (gdb) info thr
    Id   Target Id                         Frame
  * 1    Thread 0x7f9e7706a440 (LWP 21169) 0x00007f9e76a9c868 in raise () from /lib64/libc.so.6
    2    Thread 0x7f9e76a60640 (LWP 21175) 0x00007f9e76b343c7 in wait4 () from /lib64/libc.so.6
    3    Thread 0x7f9e7613d640 (LWP 21176) 0x00007f9e76b343c7 in wait4 () from /lib64/libc.so.6
    4    Thread 0x7f9e7493a640 (LWP 21179) 0x00007f9e76b343c7 in wait4 () from /lib64/libc.so.6
    5    Thread 0x7f9e7593c640 (LWP 21177) 0x00007f9e76b343c7 in wait4 () from /lib64/libc.so.6
    6    Thread 0x7f9e7513b640 (LWP 21178) 0x00007f9e76b343c7 in wait4 () from /lib64/libc.so.6
    7    Thread 0x7f9e6ffff640 (LWP 21180) 0x00007f9e76b343c7 in wait4 () from /lib64/libc.so.6
    8    Thread 0x7f9e6f7fe640 (LWP 21181) 0x00007f9e76b343c7 in wait4 () from /lib64/libc.so.6
  (gdb) p/x $pm->thread_info[0].pth_id
  $12 = 0x7f9e7706a440
  (gdb) p/x $pm->thread_info[1].pth_id
  $13 = 0x7f9e76a60640

  (gdb) set $px = *$pm->proxies
  while ($px != 0)
     printf "%#lx %s served=%u\n", $px, $px->id, $px->served
     set $px = ($px)->next
  end

  0x125eda0 GLOBAL served=0
  0x12645b0 stats served=0
  0x1266940 comp served=0
  0x1268e10 comp_bck served=0
  0x1260cf0 <OCSP-UPDATE> served=0
  0x12714c0 <HTTPCLIENT> served=0