Provide lines containing extra, unrecognized arguments

This question came up when implementing #21934: do we need to include lines with extra arguments in Descriptor#getUnrecognizedLines()? Or do we need another method Descriptor#getPartiallyUnrecognizedLines()? Or can we safely drop those extra arguments on the floor? See also TorperfResults#getUnrecognizedKeys() which is somewhat related here, but not exactly the same.

This is not super urgent, because at least the bytes of the entire descriptor are available for further processing.

The fix is also not trivial to write, because we'll have manually go through all places where extra arguments are permitted and ignored and add those lines to a list of partially unrecognized lines.

The result could be that doing nothing is the way to go. Needs more thoughts.