Adds zstd support to the btrfs program. An optional dependency on libzstd
>= 1.0.0 is added. Autoconf accepts `--enable-zstd' or `--disable-zstd' and
defaults to detecting if libzstd is present using `pkg-config'. Adds tests
for the new features based on a prebuilt btrfs image with a zstd compressed
file.

The patch is also available in my fork of btrfs-progs [1], which passes
Travis-CI with the new tests. The prebuilt binary is available there.

I haven't updated Android.mk.

[1] https://github.com/terrelln/btrfs-progs/tree/devel

Signed-off-by: Nick Terrell <terrelln@fb.com>