Rename kernel-only members of semid_ds and msgid_ds.

This deliberately breaks the API in preperation for future syscall
revisions which will remove these nonstandard members.

In an exp-run a single port (devel/qemu-user-static) was found to
use them which it did becuase it emulates system calls. This has
been fixed in the ports tree.

PR: 224443 (exp-run)
Reviewed by: kib, jhb (previous version)
Exp-run by: antoine
Sponsored by: DARPA, AFRP
Differential Revision: https://reviews.freebsd.org/D14490