apt: use its new colour support

apt v2.0.1 introduced support for coloured E:/W:/N: labels. this adds support to control it based upon our own colour control.

note that with utilities like dpkg we do not do this, but apt only uses its new colour support automatically when apt is used directly, it is not automatically enabled (per isatty()) for apt-get/aptitude (the apt developer responsible for adding colour support in response to my request for it told me that it was deliberately done like this per being customary to not change behaviour of those tools for compatibility reasons). colour errors/warnings are useful, so we want to turn it on for our use of these tools where we can.

Gbp-Dch: Short