Add validate_augeas command

This PR adds a new `validate_augeas` function, which takes a string and an Augeas lens, and validates the string against the Augeas lens. Example:

    validate_augeas("root ALL=(ALL) ALL\n", "Sudoers.lns")

The function optionally takes an array of paths which should not be found in the content.

For example, if you want to make sure your passwd content never contains  a user `foo`, you could write:

    validate_augeas($passwdcontent, 'Passwd.lns', ['$file/foo'])

Or if you wanted to ensure that no users used the '/bin/barsh' shell, you could use:

    validate_augeas($passwdcontent, 'Passwd.lns', ['$file/*[shell="/bin/barsh"]']

Like all validate_* functions, it fails compilation if the check fails. It also takes an optional 4rd argument to specify a nicer message.