allow configuration of a pre-execution hook for arbitrary conditions

[d10r]

On my notebook, I wanted automatic updates to be done only when connected to Ethernet and AC Power.
Since I didn't find a way to do this with unattended-upgrades as is, I added a config option which allows to make execution dependent on arbitrary conditions by executing a given script and checking its return value.

Usage described in README.md

As an example, that's the script I'm using it with (tailored to my system):

#!/bin/bash

set -e
set -u

ETH_CONNECTED=false
AC_CONNECTED=false

# check Ethernet connected
if [[ `cat /sys/class/net/eth0/operstate` == "up" ]]; then
  ETH_CONNECTED=true
fi

# check AC connected
if [[ `cat /sys/devices/platform/smapi/ac_connected` == 1 ]]; then
  AC_CONNECTED=true
fi

if $ETH_CONNECTED && $AC_CONNECTED; then
  exit 0
fi

exit 1

[rbalint]

In typical configurations the processes calling unattended-upgrade implement that thus I think there would be little value in adding the feature to u-u, too.
https://anonscm.debian.org/git/apt/apt.git/commit/?id=51d659e7d8cdce59f910eceeee68e2c2afdb70d4