games/xasteroids: fix build, cleanup

• replace shar EXTRACT_CMD hack with custom do-extract target to fix the build (failed since EXTRACT_ENV was introduced)
• define LICENSE and follow license obligations
• clean up port Makefile, pet portlint
• regenerate patches
• add prototypes to source code
• adopt this unmaintained port

Upstream has been contacted about the project license in the hope of
the project perhaps receiving a more conventional one.

MFH: 2026Q2
(cherry picked from commit 7c211da31554ef159a893171ac42a29555b6fc31)