Bug#1094901: modernize-sources: allow other than all or nothing

[Dan Jacobson <jidanni@jidanni.org>]

Package: apt
Version: 2.9.26
Severity: minor

$ apt modernize-sources
...
For a simulation, respond N in the following prompt.
Rewrite 3 sources? [Y/n] N

First, the N should also be inside the box, as it is a valid choice. But
then it competes with the Y for looking like the default. So use "s" to
simulate.

Then, what if the user only wants to mess with their second out of three
sources. So please say there how to pick other than just "all or
nothing".

Maybe [123][Yns]?

What about if they have more than nine? Well they must be experts
already.