When 'fscrypt status DIR' detects that a v1-encrypted directory is still
usable but its key seems to be absent, it shows the status as
"Unlocked: Partially (incompletely locked)".  But actually it can also
be the case that the directory is unlocked by another user.  Adjust the
status message accordingly.

This commit also fixes cli-tests/t_v1_policy.

The test user's user keyring is still linked into root's user keyring at
the end of the test.  This is making the test flaky, as there is a
failure that only occurs the first time it is run.  Fix the test to
restore the initial state.  This makes it consistently fail (to be fixed
by the next commit).

Use %q, in case the paths contain whitespace.  Also clean the directory
path to remove trailing slashes before appending the ".new" suffix.

Update https://github.com/google/fscrypt/issues/220

In checkEncryptable(), check whether the directory is already encrypted
before checking whether it's empty.

Also improve the error message for when a directory is nonempty.

Finally, translate keyring.ErrKeyAddedByOtherUsers and
keyring.ErrKeyFilesOpen into errors which include the directory.

Introduce filesystem.ErrEncryptionNotEnabled and
filesystem.ErrEncryptionNotSupported which include the Mount as context,
and translate the corresponding metadata/ errors into them.  Then make
these errors show much better suggestions.

Also replace lots of other filesystem/ errors with either custom types
or with unnamed one-off errors that include more context.  Fix backwards
wrapping in lots of cases.

Finally, don't include the mountpoint in places where it's not useful,
like OS-level errors that already include the path.

ErrBadOwners:
	Rename to ErrDirectoryNotOwned for clarity, move it from
	cmd/fscrypt/ to metadata/ where it better belongs, and improve
	the message.

ErrEncrypted:
	Rename to ErrAlreadyEncrypted for clarity, and include the path.

ErrNotEncrypted:
	Include the path.

ErrBadEncryptionOptions:
	Include the path and bad options.

ErrEncryptionNotSupported:
ErrEncryptionNotEnabled:
	Don't wrap with "get encryption policy %s", in preparation for
	wrapping these with filesystem-level context instead.

Also avoid mixing together the error handling for the "get policy" and
"set policy" ioctls.  Make it very clear how we're handling the errors
from each ioctl.

ErrAccessUserKeyring:
	Include the user, and fix the backwards wrapping.

ErrSessionUserKeyring:
	Include the user.

ErrKeyAdd:
ErrKeyRemove:
ErrKeySearch:
ErrLinkUserKeyring:
	Replace these with one-off unnamed errors because they are
	never checked for, and this makes it easier for the callers to
	provide better messages, e.g. fixing the backwards wrapping.

ErrMissingPolicyMetadata:
	Include the mount, directory path, and metadata path.  Also move
	the explanation into actions/ since it doesn't refer to any CLI
	command.

ErrPolicyMetadataMismatch:
	Include a lot more information.  Also start checking for
	consistency of the policy key descriptors, not just the
	encryption options.  Add a test for this.

ErrDifferentFilesystem:
	Include the mountpoints.

ErrOnlyProtector:
	Clarify the message and include the protector descriptor.

ErrAlreadyProtected:
ErrNotProtected:
	Include the policy and protector descriptors.

ErrAccessDeniedPossiblyV2:
	Make it slightly clearer what failed.  Also move the explanation
	into actions/ since it doesn't refer to any CLI command.

ErrProtectorName:
	Rename to ErrLoginProtectorName for clarity, and include the
	name and user.

ErrMissingProtectorName:
	Include the correct protector source.

ErrDuplicateName:
	Rename to ErrProtectorNameExists for clarity, and remove a level
	of wrapping by including the name directly.

ErrDuplicateUID:
	Rename to ErrLoginProtectorExists for clarity, and remove a
	level of wrapping by including the user directly.