github.com/golang/protobuf/proto has been deprecated in favor of
google.golang.org/protobuf/proto, so migrate to the non-deprecated one.

World-writable directories are not appropriate for some systems, so
offer a choice of single-user-writable and world-writable modes, with
single-user-writable being the default.  Add a new documentation section
to help users decide which one to use.

Fixes #123

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 encrypting a directory using a raw_key protector.