Allow the ND6 default route and prefix lists to be read in capability mode.

[markj]

Summary

This is needed for capsicumization of some network code. These lists
are maintained by the kernel and can be dumped by any user (e.g., with
ndp -p or ndp -r); I can't see any reason to prohibit access in cap
mode.

[oshogbo]

Why don't use casper for that (e.g. cap_sysctlbyname) ?

[markj]

In D17485#373089, @oshogbo wrote:

Why don't use casper for that (e.g. cap_sysctlbyname) ?

I considered that and decided not to for two reasons:

1. These sysctls don't have names, so the existing API isn't sufficient.
2. It seems a bit silly - what do we gain by restricting access to these read-only sysctls?

If 1) can be addressed, I'll do it, but I don't understand the threat model which makes that approach preferable.

[emaste]

Perhaps a topic for discussion at the Vendor/Dev Summit next week

[markj]

I don't need this after all: the code which uses the sysctl value performs some other actions which require a casper helper.

We also discussed this a bit at MeetBSD; Mariusz made the point that the approach of exposing whichever random sysctls happen to be needed isn't really sustainable. I'm doing some work to flesh out cap_sysctl a bit further.