Always use platform PRNG

Always use platform PRNG

Remove the fortuna and device PRNG modules and PRNG modularity, and
move the prng_os implementation into prng.c.  Remove the crypto_mod.h
requirement to implement failure-free AES256 and SHA256.  Deprecate
krb5_c_random_add_entropy() and krb5_c_random_os_entropy() and remove
their call sites.  Deprecate and ignore the -W (weak random) option to
kadmind and kdb5_util create, and stop using it in the test suite.

ticket: 9032 (new)