MINOR: debug/cli: add some debugging commands for developers

MINOR: debug/cli: add some debugging commands for developers

When haproxy is built with DEBUG_DEV, the following commands are added
to the CLI :

  debug dev close <fd>        : close this file descriptor
  debug dev delay [ms]        : sleep this long
  debug dev exec  [cmd] ...   : show this command's output
  debug dev exit  [code]      : immediately exit the process
  debug dev hex   <addr> [len]: dump a memory area
  debug dev log   [msg] ...   : send this msg to global logs
  debug dev loop  [ms]        : loop this long
  debug dev panic             : immediately trigger a panic
  debug dev tkill [thr] [sig] : send signal to thread

These are essentially aimed at helping developers trigger certain
conditions and are expected to be complemented over time.