kernel - Fix jumbo cluster buffer deadlock

* mbufjcluster_cache and mbufphdr_jcluster_cache did not have
  a nominal maintainance number set, which causes objcache to
  default to (cluster_limit / 2).  Both of these caches are fed
  from mjclmeta_cache.  The default maintainance value combined
  for these two allows mjclmeta_cache to become completely exhausted.

  The exhaustion results in an edge case when combined with the per-cpu
  caches which can deadlock the mjclmeta_cache.  The other mbuf caches
  do not have this problem because they specify maintainance divisors
  of at least 4.

* Implement kern.ipc.mjclph_cachefrac and kern.ipc.mjcl_cachefrac to
  force the two jcluster caches to return more buffers to mjclmeta_cache.
  Default to 4 and 16.

* Force all cachefrac values for all mbuf caches to not be less than 3
  to prevent sysop foot-shooting.

* Also set a fixed cachefrac of 4 for mbuf_cache, mclmeta_cache, and
  mjclmeta_cache.  The default in objcache of 2 (aka 1/2) is overkill.
  (this change is subject to review from Sephe).

Reported-by: joris
kernel/rum: Fix TX rate control. Use usb_pause_ls instead of zsleep.

* Fix TX rate control by interpreting the TX statistic counters correctly.
  Taken-From: OpenBSD

* Using usb_pause_ls instead of zsleep seems to avoid deadlocks.
csprng - don't wait for entropy for the ratectl'ed reseed
csprng - fix unused variable