DEBUG: counters: add the ability to enable/disable updating the COUNT_IF counters

DEBUG: counters: add the ability to enable/disable updating the COUNT_IF counters

These counters can have a noticeable cost on large machines, though not
dramatic. There's no single good choice to keep them enabled or disabled.
This commit adds multiple choices:
  - DEBUG_COUNTERS set to 2 will automatically enable them by default, while
    1 will disable them by default
  - the global "debug.counters on/off" will allow to change the setting at
    boot, regardless of DEBUG_COUNTERS as long as it was at least 1.
  - the CLI "debug counters on/off" will also allow to change the value at
    run time, allowing to observe a phenomenon while it's happening, or to
    disable counters if it's suspected that their cost is too high

Finally, the "debug counters" command will append "(stopped)" at the end
of the CNT lines when these counters are stopped.

Not that the whole mechanism would easily support being extended to all
counter types by specifying the types to apply to, but it doesn't seem
useful at all and would require the user to also type "cnt" on debug
lines. This may easily be changed in the future if it's found relevant.