In dumpcap, if we're being run by TShark or Wireshark, if there are no
link-layer types, just provide an empty list to our caller; let them
construct an empty list of link-layer types when they read our output.

In the code that reads that list, don't report an error if the list is
empty, rely on the caller to do so.

Have capture_opts_print_if_capabilities() do more work, moving some
functions from its callers to it.