Workaround: add dpopts to UserSwitch

Currently, slicing on the user switch breaks bandwidth limits.
We don't yet have a good way of using both of them at the same time.
I'm inclined to turn off slicing by default, but I have to think
about it and also see if any one is using it...
Workaround: add default dpopts='--no-slicing' to UserSwitch

This disables slicing by default but fixes bandwidth limits.
Eventually we want to enable both to work together, but for now
this enables one or the other depending on the dpopts setting.