bug#22512: toggle showing Info index numbers

["積丹尼 Dan Jacobson" <jidanni@jidanni.org>]

In Info mode:

 1 .. 9	Pick first ... ninth item in node's menu.
	  Every third `*' is highlighted to help pick the right number.

Please:
1. Add a way to go to the 43rd item, not just "place cursor upon the first
item and type ESC 43 CTRL+N, then RET".

Maybe model it off of:
M-g (translated from <escape> g) runs the command goto-line.

2. Add a toggle, so each item in an index now shows its number.

E.g.,
* The iplookup router::
* The manualroute router::
* The queryprogram router::
* The redirect router::
* Environment for running local transports.

Becomes
* 43 The iplookup router::
* 44 The manualroute router::
* 45 The queryprogram router::
* 46 The redirect router::
* 47 Environment for running local transports: Environment for local transports.

Don't force us to have to look into each page to learn its number.
Don't assume numbers aren't important. They may correspond to chapter
numbers of a book.