Allow default values for protocol parameters

[ralphlange]

If protocols could set default values for parameters, it would be easier to extend protocols without breaking their database API.

Use case (happened today):
For an "intelligent" modular measurement device, modules may have one or multiple inputs ("subchannels").
The existing protocol supported a module type with one input, so the "select subchannel" command was hard-coded to 1.

An upgrade to support a module with multiple inputs made the subchannel a second parameter, which requires to change all existing databases.
If the protocol were able set a default value for parameter 2, such upgrades could keep compatibilty.