Add an optional flag and a type for UUID parameters for stat taps.

For future use, with common code to parse the -z argument.

Change-Id: Ib2a11bd5c91b5ac48ced2cd2d112b97c301ff72a
Reviewed-on: https://code.wireshark.org/review/5307
Reviewed-by: Guy Harris <guy@alum.mit.edu>