MINOR: debug: make most debug CLI commands accessible in expert mode

MINOR: debug: make most debug CLI commands accessible in expert mode

Instead of relying on DEBUG_DEV for most debugging commands, which is
limiting, let's condition them to expert mode. Only one ("debug dev exec")
remains conditionned to DEBUG_DEV because it can have a security implication
on the system. The commands are not listed unless "expert-mode on" was first
entered on the CLI :

 > expert-mode on
 > help
   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 stream ...        : show/manipulate stream flags
   debug dev tkill [thr] [sig] : send signal to thread

 > debug dev stream
 Usage: debug dev stream { <obj> <op> <value> | wake }*
      <obj>   = {strm | strm.f | sif.f | sif.s | sif.x | sib.f | sib.s | sib.x |
                 txn.f | req.f | req.r | req.w | res.f | res.r | res.w}
      <op>    = {'' (show) | '=' (assign) | '^' (xor) | '+' (or) | '-' (andnot)}
      <value> = 'now' | 64-bit dec/hex integer (0x prefix supported)
      'wake' wakes the stream asssigned to 'strm' (default: current)