Socket credentials (take 2)

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

Hi,

A while ago I proposed to add credentials to sockets. Looking back at
the thread:

    http://mail-index.netbsd.org/tech-kern/2006/07/21/0002.html

I see there were no objections, yet the thread just died.

I would like to add them now.

Questions:
  - Are there any locking considerations present today that weren't
    when the diff was made, or are omitted in it?
        
  - I think "struct uidinfo" is only a member of "struct socket".
    Perhaps, if we put credentials in a socket, we can remove it from
        there, remove the "ui_uid" member, and attach it to the socket's
        credentials using kauth_cred_setdata()?
        
  - Since the credentials on the socket should not change (I think!)
    maybe we should introduce some way to mark them as a "snapshot",
        "immutable", or "static"?
        
Thanks,

-e.