Split bdev into modules: lxcnbd

Split bdev into modules: lxcnbd

The functions

	- clone_attach_nbd();
	- do_attach_nbd();
	- nbd_busy();
	- nbd_detach();
	- nbd_get_partition();
	- wait_for_partition();

move from bdev.c to lxcnbd.c. They remain static

The functions

	- attach_nbd();
	- detach_nbd_idx();
	- nbd_clonepaths();
	- nbd_create();
	- nbd_destroy();
	- nbd_detect();
	- nbd_mount();
	- nbd_umount();
	- requires_nbd();

move from bdev.c to lxcnbd.{c,h}. They all become extern.

The struct

	- struct nbd_attach_data

moves from bdev.c to lxcnbd.c.

Adapt Makefile.am to include lxcnbd.{c,h}.

The structs

       - struct bdev; /* defined in bdev.h */
       - struct bdev_specs; /* defined in lxccontainer.h */
       - struct lxc_conf; /* defined conf.h */

are forward declared/put as incomplete types into lxcnbd.h as the functions
associated with nbd need access to it.

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>