bug#44895: Add Info-copy-permalink

[積丹尼 Dan Jacobson]

c runs the command Info-copy-current-node-name.

That's great but not powerful enough.

Let's say I am reading
(info "(emacs) Optional Mode Line")
and stumble upon the fact that
   On graphical displays, the mode line is drawn as a 3D box.

I think, "This is great. I'll be sure to tell Mrs. Nerblefield. She'll
switch editors right away."

Well, most other programs' manuals have a "share" or "copy permalink"
button, most alas except for emacs.

> We don't keep track of our URLs. Google is your friend.

Sure, but I'll probably end up finding Bill Snortfield's 1989 copy of
that page, and sending her the link to that.

> Well, find the search result with "gnu.org" in it.

Yeah, but there are more than one.

> Well then don't use Google. Just copy and paste the page's text, and
> send her that.

Yes, but she used to work at Wikipedia, so she will ask for proof.

> Well, go to gnu.org and put the title in the search box. Maybe that
> will find the link you need.

Maybe, but how about instead, along with Info-copy-current-node-name,
there was a command, Info-copy-current-node-permalink, or just
Info-copy-permalink. that would copy it for us?

> We don't hardwire https URLs into the manual. Sorry.

Yes you do. (info "(emacs) Distrib") has plenty. And that page even
knows about itself:
   (1) This manual is itself...

As a bonus, mid-page anchor # fragments could even go directly to the
places in the page there online, just like they do in the offline manual.

emacs-version "27.1"