The exclude by attribute function is broken on non-Darwin POSIX: linux and freebsd.
This is because those xattrs must be prefixed by a legal namespace. The old xattr
library implicitly appended the user namespace to the xattr, but the current
official go pkg does not (which is just as well).
Also fix the test to remove the discordant old xattr dependency and provide
test cases for both darwin and non-darwin POSIX.
Zstd compression can be enabled by providing `-zstd` or `-zstd-level <level>`
to `init`, `add`, or `backup`. With `-zstd` the compression level will be
`default`, and with `-zstd-level` the level can be any of `fastest`, `default`,
`better`, or `best`.
highwayhash 1.0.1 contains a bug leading to incorrect hashes on arm64 machines.
The 1.0.1 version is retained in github.com/gilbertchen/highwayhash so the hash
can be checked again if a mismatch is detected by 1.0.2.
