targets/xorg: Downgrade some extra packages

mesa also provides some other packages (e.g. -dev and -dbg packages),
make sure we also downgrade the ones that break when we try to
install them.

See http://packages.ubuntu.com/source/precise/mesa for a full list,
a number of them probably work fine with more recent GL libraries
and may not need downgrading (e.g. libgbm), so we do not downgrade
them.
Merge pull request #922 from drinkcat/mesa-add-missing-pin

targets/xorg: Downgrade some extra packages
targets/xorg: lts-saucy will disappear soon, use lts-trusty
Merge pull request #925 from drinkcat/xorg-precise-lts-trusty

targets/xorg: lts-saucy will disappear soon, use lts-trusty
targets/audio: Fetch libsbc 1.1

libsbc 1.0 was provided by raring, which is not provided by mirrors
anymore.
install_mirror_package: When arch is set, install both native and arch

This is necessary as dpkg multiarch does not allow mismatching versions
on different architectures (e.g. you cannot have libsbc1-1.0:i386 and
libsbc1-1.1:amd64 installed at the same time).

Even --force-all in dpkg does not allow that.

We fix the problem by installing both packages at the same time in
install_mirror_package.
test/tests/30-audio: Test that install_mirror_package is able to update libsbc1.

Force installing a newer version, and see if it gets downgraded.
tests/30_audio: Improved install_mirror_package test
Merge pull request #933 from drinkcat/libsbc-fix-version

targets/audio: Fetch libsbc 1.1
tests/18-upgrade: Remove quantal and raring tests

They are gone. Forever. :-(
saucy unsupported, add utopic

Still disappointed they didn't choose undead unicorn as codename...
tests/18-upgrade: install x11 instead of xephyr

x11 pulls in xorg, that sometimes installs alternative xorg lts packages.
xephyr does not fetch anything from mirrors anymore.
Licensing cleanup: require CLA for PRs

o hai thar, lawyercats!
Merge pull request #934 from drinkcat/remove-quantal-raring-test

tests/18-upgrade: Remove quantal and raring tests
enter-chroot: If XAUTHORITY is not set, make a guess

Useful when enter-chroot is called from udev/upstart, see #929.
Merge pull request #942 from drinkcat/enter-chroot-guess-xauth

enter-chroot: If XAUTHORITY is not set, make a guess
Fix licensing and update contributor instructions
Set UID_MIN to 1001 to make sure that useradd does not automatically add users with UID 1000.

This guarantees that UID 1000 is available when post-common adds the main user at the end of the first run of crouton.

Protect GID 1000 in the same way.

Fixes a bug when xbmc is selected during the Arch install (the xmbc user created by the package steals UID 1000).
arch: installer
arch: enter-chroot: source /etc/profile.

Workaround for bug in su/bash, where /etc/profile is not sourced,
even though su is called with '-' (login shell).
See: https://bugs.archlinux.org/task/31831
This is necessary to get /usr/local/bin in the PATH, and setup the
correct locale, among other things. This is not necessary in Ubuntu,
as these are set by /etc/pam.d/su