Plug some kernel memory disclosures via kevent(2).

The kernel may register for events on behalf of a userspace process,
in which case it must be careful to zero the kevent struct that will be
copied out to userspace.

Reviewed by: kib
MFC after: 3 days
Security: kernel stack memory disclosure
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18317