implement c->reboot(c) in the api.

Also if the container is not running, return -2.  Currently
lxc-stop will return 0, so you cannot tell the difference
between successfull stopping and noop.

Per stgraber's email:

 - Remove lxc-shutdown
 - Change lxc-stop so that:
   * Default behaviour is to call shutdown(), wait 15s for STOPPED, if
not STOPPED, print a message to the user and call stop() [ NOTE:
actually 60 seconds per followup thread]
   * We have a -r option to reboot the container (with proper check that
the container indeed rebooted within the next 15s)
   * We have a -s option to shutdown the container without the automatic
fallback to stop()
   * Add a -k option allowing a user to just kill a container
(equivalent to old lxc-stop, no shutdown() call and no delay).

and update manpages.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>