Do the "-z help" processing separately, and do it for Wireshark as well.

Yes, Wireshark does a bunch of GUI stuff, and then takes the window down
before printing the help, but the same is true for some command-line
error messages as well.

Change-Id: Id501468416c83308e4c0a9e7a66116d8d33a9d84
Reviewed-on: https://code.wireshark.org/review/5317
Reviewed-by: Guy Harris <guy@alum.mit.edu>