add checkpoint/restore to the API

This adds support for the new checkpoint/restore API functions.
I think it would be better if we include <lxc/version.h> and decorate those two with something like below

#if LXC_VERSION_MAJOR >= 1 && LXC_VERSION_MINOR >= 1
....
#endif

otherwise one cannot use stable-1.0 with the go bindings. 

Alternatively we can branch again and declare v3 of bindings LXC 1.1+ compatible.