[arswitch] extend the debug support to be configurable at runtime.

• remove the DEBUG ifdef; defining it is too far reaching throughout the whole system;
• add a bitmask in the softc for controlling debugging;
• .. enable said debugging as a sysctl;
• add bitmaps for register access, reset and vlans.

TODO:

• Now that the debug statements are configurable, we definitely could do with more debugging
• Move the debugging into the top-level etherswitch driver and have sub-drivers obey.