implement the backend of the sysctls that report pool cache info.

implement the backend of the sysctls that report pool cache info.

KERN_POOL_CACHE reports info about the global cache info, like how long
the lists of cache items the cpus build should be and how many of these
lists are idle on the pool struct.

KERN_POOL_CACHE_CPUS reports counters from each each. the counters
are for how many item and list operations the cache has handled on
a cpu. the sysctl provides an array of ncpusfound * struct
kinfo_pool_cache_cpu, not a single struct kinfo_pool_cache_cpu.

tested by hrvoje popovski
ok mikeb@ millert@
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