Rename stat_cmd_args.[ch] to stat_tap_ui.[ch].

The intent is to handle more than just command-line arguments; reflect that.

Change-Id: Ia10efda85a9d11c6579d1bec6f789cee30d9e825
Reviewed-on: https://code.wireshark.org/review/5304
Reviewed-by: Guy Harris <guy@alum.mit.edu>