ip6addrctl(8): Teach ip6addrctl to attach and run itself in a jail

[zlei]

Summary

This will make it easier to manage address selection policies in vnet
jails, especially for those light weighted OCI containers or slim jails.

Requested by: dfr
MFC after: 1 week
Relnotes: yes

Test Plan

# ip6addrctl
...
# jail -c name=foo vnet persist
# ip6addrctl -j foo
no source-address-selection policy is installed
# echo "::/0 40 1" > /tmp/ip6addrctl.conf
# ip6addrctl -j foo install /tmp/ip6addrctl.conf
# ip6addrctl -j foo
Prefix                          Prec Label      Use
::/0                              40     1        0

[zlei]

usr.sbin/ip6addrctl/ip6addrctl.8

112

I guess this should be refined.

[zlei]

usr.sbin/ip6addrctl/ip6addrctl.8

72

That means ip6addrctl show -j foo does not work as intended.

[dfr]

usr.sbin/ip6addrctl/ip6addrctl.8

72

Perhaps clarify with an example, e.g. 'ip6addrctl -j foo show'?

112

I think the wording is ok - it seems clear that the file is read and processed on the host but applied in the jail.

[markj]

usr.sbin/ip6addrctl/ip6addrctl.c

138

[zlei]

usr.sbin/ip6addrctl/ip6addrctl.8

72

The SYNOPSIS section already has this usage. I intend to emphasize the words should precede, or users may make wrong usage, for example ip6addrctl show -j foo will succeed but actually operate in current jail.

usr.sbin/ip6addrctl/ip6addrctl.c

112

That means ip6addrctl show -j foo does not work as intended.

Maybe it is better to strictly check the arg count ? For example ip6addrctl add accept exactly 3 extra arguments ? i.e

---	if (argc < 4)
+++	if (argc != 4)

138

Good catch, I copy-pasted the declaration of this function.

[zlei]

usr.sbin/ip6addrctl/ip6addrctl.8

72

That means ip6addrctl show -j foo does not work as intended.

With D48701, the above will be invalid, and ip6addrctl will prompt user the right usage.

If you think that is the right approach, I can rebase this.

[dfr]

usr.sbin/ip6addrctl/ip6addrctl.8

72

Works for me - I approved D48701.

[zlei]

Rebase onto D48701.