Fortuna as default PRNG

Fortuna as default PRNG

Rewrite prng_fortuna.c to much more closely match the description of
Fortuna in chapter 9 of Cryptography Engineering.  Add a facility to
get OS entropy and implement it for Unix and Windows (not yet tested
on Windows) to replace prng/fortuna/entropy.c.  Rewrite the test
harness to always ensure stable output and perform a statistical test
on the predictable internal state resulting from the stable-output
tests.

ticket: 6874

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24652 dc483132-0cff-0310-8789-dd5450dbe970