Add systemd unit for lxc.net

This is the equivalent of the upstart lxc-net.conf to set up the LXC bridge.

This also drops "lxc.service" from tarballs. It is built source which depends
on configure options, so the statically shipped file will not work on most
systems.

https://launchpad.net/bugs/1312532
systemd: Ensure action() is defined

If /etc/rc.d/init.d/functions is not present or does not define an action()
function, provide a simple fallback using "echo".
systemd: Load AppArmor profiles if necessary/supported

On Ubuntu we need to set up the AppArmor profiles also under systemd. Add a new
helper "lxc-apparmor-load" and integrate it into lxc.service.
Install systemd units for Ubuntu