inet6: Simplify in6ifa_ functions

And fix indentation of in6ifa_llaonifp() while I'm here.
usbconfig(8): Sync with FreeBSD (decodes bDeviceClass/bInterfaceClass).
Mark some legacy XSI functions as no longer present in _XOPEN_SOURCE=700.

This applies if _XOPEN_SOURCE=700 is explicitly requested. Our default
environment still exposes the prototypes, of course.