Use getopt #8

Use getopt to handle dpu's command-line options.
No change in functionality, but this will make it easier to handle other command-line options as they're implemented (see: #5 #6 #7).

Holy shit, is this just replacing the existing functionality?! But only counting to 10?! Gah!

This counts as high as you want it to; the 0-9 option handling is a base 10 decoder.
(Because of how getopt works, you can say -1 -2 -3 and it will do the same thing as either the old or new version does with -123. I'm not sure whether that's a stealth feature, or a bug I don't care about.)

I misread, get it now

I hate this

I refuse to allow this to meet the need of "use getopt like a real project", despite meeting the need of "use getopt"