Set change flag from SERVER.WRITEFILE
Support $path in file source/template attrs

The $path `variable' will be replaced with the path of the file
resource, making it trivially easy to set up parallel directory trees
per policy:

    file "/etc/motd" {
        source: "/cfm/files/baseline/etc/motd"
    }

is now equivalent to (the less-typo prone):

    file "/etc/motd" {
        source: "/cfm/files/baseline/$path"
    }