Split bdev into modules: lxcrsync

Split bdev into modules: lxcrsync

The functions:

        - do_rsync();
        - rsync_delta();
        - rsync_delta_wrapper();
        - rsync_rootfs();
        - rsync_rootfs_wrapper();

and the structs

        - struct rsync_data;
        - struct rsync_data_char;

move from bdev.{c,h} to lxcrsync.{c.h}. All functions previously declared as
static become public.

lxcrsync.{c,h} should allow for a reasonable amount of abstraction regarding
our rsync functions. Some of the functions could easily be abstracted.

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

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