autostart: Define lxc.start.* and lxc.group

First patch in the set of changes required for container autostart.

This commit adds the new configuration keys and parsers that will then
be used by lxc-start and lxc-stop.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Dwight Engen <dwight.engen@oracle.com>