Don't declare union semun in userspace unless _WANT_SEMUN is defined.

POSIX explicitly states that the application must declare union semun.
This makes no sense, but it is what it is. This brings us into line
with Linux, MacOS/Darwin, and NetBSD.

In a ports exp-run a moderate number of ports fail due to a lack of
approprate autotools-like discovery mechanisms or local patches. A
commit to address them will follow shortly.

PR: 224300, 224443 (exp-run)
Reviewed by: emaste, jhb, kib
Exp-run by: antoine
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14492