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.

Test getting global, filesystem, and unencrypted directory status when
the filesystem is or isn't set up for fscrypt.