Linux v5.4 and later supports v2 encryption policies.  These have
several advantages over v1 encryption policies:

- Their encryption keys can be added/removed to/from the filesystem by
  non-root users, thus gaining the benefits of the filesystem keyring
  while also retaining support for non-root use.

- They use a more standard, secure, and flexible key derivation
  function.  Because of this, some future kernel-level fscrypt features
  will be implemented for v2 policies only.

- They prevent a denial-of-service attack where a user could associate
  the wrong key with another user's encrypted files.

Prepare the fscrypt tool to support v2 encryption policies by:

- Adding a policy_version field to the EncryptionOptions, i.e. to the
  config file and to the policy metadata files.

- Using the kernel-specified algorithm to compute the key descriptor for
  v2 policies.

- Handling setting and getting v2 policies.

Actually adding/removing the keys for v2 policies to/from the kernel is
left for the next patch.

In preparation for introducing support for the new filesystem-level
keyrings, move the existing user keyring management code from
security/keyring.go and crypto/crypto.go into a new package, 'keyring'.

This package provides functions AddEncryptionKey, RemoveEncryptionKey,
and GetEncryptionKeyStatus which delegate to either the filesystem
keyring (added by a later patch) or to the user keyring.  This provides
a common interface to both types of keyrings, to the extent possible.

Use the new name for fscrypt constants and structures which have been
given a new name.

Also use the named constant for the DIRECT_KEY fscrypt policy flag.

No change in behavior.  This is just preparing for future work.

These were found by a combination of manual review and a custom script
that checks for common errors.

Also removed an outdated sentence from the comment for setupBefore().

Use the golang library for the hashing function instead of the reference
C implementation. This removes the dependancy on libargon2. As we are no
longer doing our own error checking, we also eliminate those tests.