random(4): deduplicate explicit_bzero() in harvest

random(4): deduplicate explicit_bzero() in harvest

Pull the responsibility for zeroing events, which is general to any
conceivable implementation of a random device algorithm, out of the
algorithm-specific Fortuna code and into the callers.  Most callers
indirect through random_fortuna_process_event(), so add the logic there.
Most callers already explicitly bzeroed the events they provided, so the
logic in Fortuna was mostly redundant.

Add one missing bzero in randomdev_accumulate().  Also, remove a redundant
bzero in the same function -- randomdev_hash_finish() is obliged to bzero
the hash state.

Reviewed by:	delphij
Approved by:	secteam(delphij)
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D20318