[RFE] Attempted to restore attributes from specification

[vbatts]

like the -t and -u flags of the mtree cli.

     -t                 Modify the modified time of existing files, the device
                        type of devices, and symbolic link targets, to match
                        the specification.
     -u                 Modify the owner, group, permissions, and flags of
                        existing files, the device type of devices, and sym-
                        bolic link targets, to match the specification.  Cre-
                        ate any missing directories, devices or symbolic
                        links.  User, group, and permissions must all be spec-
                        ified for missing directories to be created.  Note
                        that unless the -i option is given, the schg and
                        sappnd flags will not be set, even if specified.  If
                        -m is given, these flags will be reset.  Exit with a
                        status of 0 on success, 2 if the file hierarchy did
                        not match the specification, and 1 if any other error
                        occurred.

[vbatts]

As part of this, I'm going to change the behavior of the xattr keyword.
Rather than having the value being a sha1 digest of the value, I'm instead going to right the actual value but base64 encoded.
I was two ways about this, but after chatting with @mjg59 I'm going to go with base64, so the values could be restored to a file.