Rework init scripts

This commit is based on the work of:
    Signed-off-by: Michael H. Warfield <mhw@WittsEnd.com>

A generic changelog would be:
 - Bring support for lxcbr0 to all distributions
 - Share the container startup and network configuration logic across
   distributions and init systems.
 - Have all the init scripts call the helper script.
 - Support for the various different distro-specific configuration
   locations to configure lxc-net and container startup.

Changes on top of Mike's original version:
 - Remove sysconfig/lxc-net as it's apparently only there as a
   workaround for an RPM limitation and is breaking Debian systems by
   including a useless file which will get registered as a package provided
   conffile in the dpkg database and will therefore cause conffile prompts
   on upgrades...
 - Go with a consistant coding style in the various init scripts.
 - Split out the common logic from the sysvinit scripts and ship both in
   their respective location rather than have them be copies.
 - Fix the upstart jobs so they actually work (there's no such thing as
   libexec on Debian systems).

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>