srd: Support for one or more optional probes.

srd: Support for one or more optional probes.

In the protocol decoder you always get all required probes, then _all_
optional probes in the list of probes in the decode() call.

Example:

 (r1, r2, r3, o1, o2, o3, o4) = pins

In this case r1-r3 are required probes, o1-o4 are optional probes.
However, the value of valid/used/specified probes will be 0 or 1,
whereas the value of probes that were not specified/assigned by the user
will be (at the moment) 42.

The PD can check for a valid probe like this:

  if p in (0, 1):
    ...

Or check for an invalid probe:

  if (p > 1):
    ...

The value of 42 could change to be -1 or None later.