The random number generator was not generating sufficient entropy by

The random number generator was not generating sufficient entropy by
default, resulting in weak random numbers for a short period of time after
a machine is first booted.

* Change the entropy default for all interrupts except the clock interrupt
  from off to on.

* Greatly reduce the overhead of the interrupt entropy code so even high
  performance interrupts can call it.

* Instead of calculating the entropy at the time of the interrupt,
  introduce a new rate-limited kernel thread which the interrupt code can
  simply schedule.  The new thread will be responsible for adding the
  entropy.

  This thread will rate-limit based on the amount of entropy that has been
  built up.  The more entropy we have, the lower the thread's frequency of
  operation.  Currently the limit is set to 25 hz.

* Use the TSC in addition to the normal time calculation when introducing
  entropy.

Note that during tests it was found that a new ssh connection tends to
'eat' all available entropy.  This isn't actually true, it's really the
entropy calculation itself which is not quite correct, but I am duely noting
the issue here.

Reported-by: "Simon 'corecode' Schubert" <corecode@fs.ei.tum.de>