vfs: stricter validation for flags passed to namei in cn_flags

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.