API pruning

Hi Tim--

here are two proposed changes to the libpsl API that make more sense (i think) from a library user perspective.

I'd be happy to submit future patches via the mailing list for review if you're ok with that -- i don't particularly like the github webinterface, and i prefer to be able to work offline with my mail user agent anyway.

but if you prefer github pull requests, i am also fine with continuing to do things this way.

thanks for your work on this project.