in6.h: expose s6_addr* definitions to user level

The only element of of in6_addr that is specified in RFC 3493 or
in POSIX.1-2017 is s6_addr, implemented via a #define to a union
member. However, FreeBSD and other BSD systems have additional
definitions for the other union members, s6_addr{8,16,32} which
are defined for the kernel and loader. Some Linux applications
also use them, and they seem to be allowed by the RFC and POSIX.
Remove the current ifdefs, exposing the additional fields to user
level, and replace with #if __BSD_VISIBLE. Add an explanatory
comment expanding on the previous "nonstandard" comment.

MFC after: 1 week
Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D44979