Introduce send-dump.[ch] which implements a new btrfs_send_ops to
exam and output all operations inside a send stream.

It has a better output format than the old and no longer compilable
send-test tool, but still tries to be script friendly.

Provides the basis for later "inspect-internal dump-send" command.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
[ drop escaping call from __print_dump for now, drop gcc warning tweaks,
  replace empty format strings with NULL ]
Signed-off-by: David Sterba <dsterba@suse.com>