Bug#599007: Ctrl-C inhibited even in subshells

Ctrl-C not simply terminating apt any more might be good
from a package installation consistency view.

But that should be reverted for any subshells that get opened!


When there's a merge conflict in some config file,
opening the relevant files in vim shows unexpected behaviour --
eg. canceling a commandline with Ctrl-C doesn't work any more,
and this breaks (at least my) muscle memory.