destroy: implement in the api

destroy: implement in the api

This requires implementing bdev->ops->destroy() for each of the backing
store types.  Then implementing lxcapi_clone(), writing lxc_destroy.c
using the api, and removing the lxc-destroy.in script.

(this also has a few other cleanups, like marking some functions
static)

Changelog:
	fold into destroy: fix zfs destroy
	destroy: use correct program name in help

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