Removing dpkg conffile backgrounding prompt support

[Guillem Jover <guillem@debian.org>]

Hi!

I'd like to know if people would strongly miss being able to
background dpkg on the conffile prompt (‘Z’), instead of starting a
new subshell. The latter is the default with most modern frontends
(APT based).

I personally find the background support annoying and confusing when
one is used to expect a subshell. It would also allow to properly fix
bug 60329, which I think would be nice.

Otherwise I'd like to remove it in the near future.

thanks,
guillem