PRNG cleanups

[ghudson@MIT.EDU]

Based on discussion last September, I've written up a project proposal
for changes to the PRNG logic in krb5, and started a one-week review:

http://k5wiki.kerberos.org/wiki/Projects/PRNG_Cleanup

The basic proposed steps are:

* Rewrite the Fortuna PRNG implementation and make it the default.

* Immediately reseed the Fortuna generator on OSRAND and TRUSTEDPARTY
  entropy inputs; other inputs go into the accumulator pools.

* Make the Fortuna PRNG fail out if it can't productively seed its
  generator (this shouldn't ever happen if it's possible to get OS
  entropy).

* Drop the Yarrow PRNG implementation.

* Add a PRNG implementation which just gathers OS entropy.

* Eliminate some of the calls to add entropy inputs where they are not
  needed or useful.