main.cpp: straighten the getopt/optind adjustment before openfiles

The pulseview(1) application accepts input file specs either as argument
to the -i option, or as positional arguments after the command line options.

Simplify the logic which collects remaining non-option command line words
after getopt(3) is done. As a byproduct the application code no longer
manipulates a variable that is owned by an external library.