vfs: stricter validation for flags passed to namei in cn_flags
namei de facto expects that the naimeidata object is properly initialized,
but at the same time it mixes consumer-passable and internal flags, while
tolerating this part by explicitly clearing some of them.
Tighten the interface instead.
While here renumber the flags and denote the gap between the 2 variants.
Try to piggy back th renumber on the just bumped __FreeBSD_version.