into one if a system has an awesome getentropy().  In that case it
is valid to totally throw away the rsx state in the child.  If the
getentropy() is not very good and has a lazy reseed operation, this
combining is a bad idea, and the reseed should probably continue to
use the "something old, something new" mix.  _rs_allocate() can
accomodate either method, but not on the fly.
ok matthew