Avoid copying out kernel pointers from msgctl(IPC_STAT).

While this behaviour is harmless, it is really just an artifact of the
fact that the msgctl(2) implementation uses a user-visible structure as
part of the internal implementation, so it is not deliberate and these
pointers are not useful to userspace. Thus, NULL them out before
copying out, and remove references to them from the manual page.

Reported by: Jeffball <jeffball@grimm-co.com>
Reviewed by: emaste, kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25600