Implement RFC 027 File Verification

This implements usable-suppliable file content verification per RFC
027. Users can supplie a block, string, or symbol to the `verify`
resource attribute.  Blocks will be called, string will be executed as
shell commands (respecing the same options as not_if and only_if), and
symbols can be used to access built-in registered validations.
The `&@block` should be just `@block`.