games/supertux fails to start with an error of not finding the datadir. It
turns out it looks in ./share/supertux2, so running it in /usr/local works
(thanks to Leonid Bobrov for the hint). The diff below fixes this by selecting
the correct datadir.
Also a few other small fixes/updates:
* The old homepage just forwards to https://www.supertux.org/. Since the old
one is http and not https, just switch to https://www.supertux.org/.
* update WANTLIB from png to png16 as it came out of make port-lib-depends-check
* make update-plist removed entries for intermediary directories to
share/icons/hicolor/scalable/apps/supertux2.svg. Please let me know if
there's a reason to keep them anyway.