lxccontainer: add reboot2() API extension

lxccontainer: add reboot2() API extension

This adds reboot2() as a new API extension. This function properly wait until a
reboot succeeded. It takes a timeout argument. When set to > 0 reboot2() will
block until the timeout is reached, if timeout is set to zero reboot2() will
not block, if set to -1 reboot2() will block indefinitly.

The struct state_client gets rename to lxc_state_client since it's more in line
with other declarations. It also gets moved from the lxc_handler to the
lxc_conf struct so that the state clients waiting for reboots don't get
deallocated on reboot since the handler is deallocated on reboot.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>