Add libbtrfsutil

Add libbtrfsutil

Currently, users wishing to manage Btrfs filesystems programatically
have to shell out to btrfs-progs and parse the output. This isn't ideal.
The goal of libbtrfsutil is to provide a library version of as many of
the operations of btrfs-progs as possible and to migrate btrfs-progs to
use it.

Rather than simply refactoring the existing btrfs-progs code, the code
has to be written from scratch for a couple of reasons:

* A lot of the btrfs-progs code was not designed with a nice library API
  in mind in terms of reusability, naming, and error reporting.
* libbtrfsutil is licensed under the LGPL, whereas btrfs-progs is under
  the GPL, which makes it dubious to directly copy or move the code.

Eventually, most of the low-level btrfs-progs code should either live in
libbtrfsutil or the shared kernel/userspace filesystem code, and
btrfs-progs will just be the CLI wrapper.

This first commit just includes the build system changes, license,
README, and error reporting helper.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>