dbus-util: let's take it down a notch when converting file mode to string

dbus-util: let's take it down a notch when converting file mode to string

I'm definitely a fan of precision, but in this case it's a bit too much:

    $ systemd-run --unit=test --socket-property=ListenFIFO=/tmp/foo --socket-property=SocketMode=0644 true
    $ systemctl cat test.socket
    # /run/systemd/transient/test.socket
    # This is a transient unit file, created programmatically via the systemd API. Do not edit.
    [Unit]
    Description=/usr/bin/true

    [Socket]
    ListenFIFO=/tmp/foo
    SocketMode=0000000000000000000000000000000000000644