Way fast to compress/decompress and produce smaller archives (compared to deflate/gzip in general), but only supported in kernels 5.9+. Notice we fallback to the default "--compress=gzip" in case kernel does not support zstd. Special thanks to Clayton (craftyguy) for the kernel version test. Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>