Description of problem:
Unfortunately while working on this issue:
  https://fedorahosted.org/system-config-printer/ticket/14

I have realised that we don't get to pass through some optional parameters when using PolicyKit.

The available optional parameters for CUPS-Get-Devices are:

1. limit, an integer, specifying the maximum number of devices to return
2. include-schemes, a list of strings specifying the inclusion list for returned device URI schemes
3. exclude-schemes, a list of strings specifying the exclusion list for returned device URI schemes

The include-schemes and exclude-schemes parameters are mutually exclusive.

There are currently no others.

Version-Release number of selected component (if applicable):
cups-pk-helper-0.0.4-7.fc12.x86_64

Additional info:
I don't think it's possible to have optional parameters with D-Bus so we need some way to signal whether the parameters should be used or not -- perhaps just '0' for no limit and empty list for the other two.  In other words, include-schemes would only be used if non-empty, otherwise exclude-schemes.

This is an API break so will need either a new interface name (can we do that?) or new policy mechanism method name.