Overhaul handling of interfaces

Several significant changes:

1. UCI SQM configuration has been removed. Only tc is used to modify bandwidths.
2. Interface direction configuration has been removed because it is now redundant.
3. Interfaces can now be specified as-is - either regular interfaces or IFB interfaces.
4. Each configured interface is now checked as follows:
    - If upload, check interface as-is for an SQM instance
    - If download, and interface is not already specified for upload, check interface as-is for an SQM instance
    - If download and no SQM instance found for interface as-is, check for an SQM instance on a corresponding IFB interface