The latest version of gofmt flattens the nested lists in comments in
crypto.go and filesystem.go.  According to
https://go.dev/doc/comment#mistakes, "Go doc comments do not support
nested lists".  However, that page also mentions that a workaround is to
use different list markers for each level.  Do that.

ErrKeyLock:
	Rename to ErrMlockUlimit for clarity.

ErrGetrandomFail:
ErrKeyAlloc:
ErrKeyFree:
ErrNegativeLength:
	Replace these with one-off unnamed errors because these were all
	returned in only one place and were never checked for.  Also
	these were all either wrapped backwards or discarded an
	underlying error, so fix that too.

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.

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Also removed an outdated sentence from the comment for setupBefore().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