kernel - Increase IFQ_MAXLEN from 50 to 250

* IFQ_MAXLEN is used as the default for numerous pseudo-network drivers.
  The value 50 is just too low.  Increase to 250.
kernel - Fix several bugs in FAIRQ

* Fix several possible overflows due to high-valued machclk_freq constants
  and uint's that should have been uint64's.  Among other things this fixes
  bandwidth calculations that could previously get into weird states.

* Refactor the fairq_selectq() routine to fix numerous cases where the
  head of the queue could get advanced multiple times without pulling a
  packet off the queue, causing packets in queues to be excessively
  delayed.

Both of these were rather serious issues.  Operation is far smoother with
the bugs fixed.