Implement RFC 027 File Verification

This implements user-suppliable file content verification per RFC
027. Users can supply a block, string, or symbol to the `verify`
resource attribute.  Blocks will be called, string will be executed as
shell commands (respecting the same options as not_if and only_if), and
symbols can be used to access built-in registered validations.
Graa.  You have no idea how many misspellings of "verification" I had to fix before submitting this and somehow I still missed this.  Will fix in next push. Thanks!