btrfs-progs: introduce rescue command group

Add an empty 1st level command namespace that will collect specialized
recovery tools like chunk-recover, zero-log, select-super and similar.

Signed-off-by: David Sterba <dsterba@suse.cz>
btrfs-progs: move chunk-recover to rescue group

Signed-off-by: David Sterba <dsterba@suse.cz>
btrfs-progs: separate command ant implementation of chunk-recover code

The command has been moved and we should rename the files accordingly,
so the entry point is now in cmds-rescue.c and the core functionality
in it's own file.

Return codes of btrfs_recover_chunk_tree have been simplified not to
require a define and another file for defintion.

Signed-off-by: David Sterba <dsterba@suse.cz>