This patch migrates the xnintr hits statistics to the new xnstat
subsystem and introduces per-CPU runtime statistics to xnintr entities.
The latter allows to differentiate between ISR and thread activity under
/proc/xenomai/stat.

For querying the xnintr statistics, a scalable service based on IRQ list
revisions is introduced (xnintr_query), just like it already exists for
threads.

Accounting model:
 - hits are only counted if the ISR returns XN_ISR_HANDLED
 - runtime is accounted over each ISR invocation, independent of its
   return value

As a hopefully not critical side effect, the native intr services now
only reports non-zero hit counters if XENO_OPT_STATS in enabled. This
fact is documented in the rt_intr_info structure.

Jan