bug#26820: 26.0.50; DNS mode and IPv6 reverse zones

[Peder O. Klingenberg]

Last week I was trying to set up reverse delegations for some newly
assigned IPv6 address space.  It was... painful.

IPv6 addresses are ugly enough in their normal format, but when used in
a reverse zone, they need to be converted to individual nibbles
separated by dots, reversed, and have .ip6.arpa. appended.  The result
is less than human friendly.

So I've written a new feature for dns-mode.el that allows me to mostly
relate to the normal IPv6 format, and convert automatically to the
nibble format.  I can for instance do this in a (part of a) zone file,
where I'm only interested in defining hosts in the last 16 bits of the
address:

$ORIGIN 2a0a:3dc0:10::/112
::1/-112 IN PTR www.example.com.

And then, by pressing C-c C-e on each of the IPv6 addresses, end up with
the necessary

$ORIGIN 0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.1.0.0.0.c.d.3.a.0.a.2.ip6.arpa. 
1.0.0.0 IN PTR www.example.com.

Assuming I'm doing this as I type, I can then yank the last
standard-format address, do a minimal edit, C-c C-e again, and continue
typing to define the next PTR record.

I'm hoping this sounds useful to other people as well.  I'll pass along
the patch as soon as I get a bug# to attach to it. :)