Some improvements for rubyversion.mk:

- Set RUBY_API_VERSION after RUBY_VERSION has decided.
- Change old RUBY_DOCDIR and RUBY_EXAMPLESDIR to RUBY_DOC and RUBY_EG in
  comment.
- Fix shared libraries PLIST to support Mac OS X with introducing RUBY_SLEXT:
Shared library => .dylib
Extension library => .bundle
- Improve PRINT_PLIST_AWK to handle new shared libraries.

No functional change shoud be done and fix PR pkg/44050.

(taca)