Big batch of KAUTH_GENERIC_ISSUSER removal

[Elad Efrat <elad%NetBSD.org@localhost>]

Hi,

The final version of what I would like to check in is available here:

    http://www.netbsd.org/~elad/issuser.diff

(it's ~100k so I didn't attach it.)

It contains both earlier diffs, plus some similar, mechanical changes
to file-system code and changes to the kauth(9) man-page.

It will not be committed before 2/22 so take your time reviewing.

Elad