r1492: Rework our random number generation system.

r1492: Rework our random number generation system.

On systems with /dev/urandom, this avoids a change to secrets.tdb for every fork().

For other systems, we now only re-seed after a fork, and on startup.
No need to do it per-operation.  This removes the 'need_reseed'
parameter from generate_random_buffer().

Andrew Bartlett
(This used to be commit 36741d3cf53a7bd17d361251f2bb50851cdb035f)