kgio 2.9.3 - minor cleanups and fixes

Most notably there's a new RB_GC_GUARD to fix an occasional segfault at
load time for Ruby 1.8 users thanks to Petr Novodvorskiy for reporting
the issue.

There's been a new mailing list for a few months now at:

    kgio-public@bogomips.org

No subscription is required, but if you wish to subscribe
(existing librelist subscribers must resubscribe)

    kgio-public+subscribe@bogomips.org

There's also a bunch of code cleanups and packaging
cleanups.  Our website is faster now thanks to olddoc:

    http://bogomips.org/kgio/

shortlog:

      test/lib_read_write: fix trywritev blocking test
      new mailing list info: kgio-public@bogomips.org
      relax license to allow LGPLv2.1 or later
      gemspec: modernize for more recent rubygems
      doc: switch to olddoc
      remove references to rubyforge and ancient Rubies
      TODO: updates for the future of kgio
      tryopen: add RB_GC_GUARD for Ruby 1.8
      LICENSE: prefer URL to FSF address
      cleanup packaging harder
      fixup -Wshorten-64-to-32 warnings
      cleanup: avoid shadowing rb_str_subseq
      cleanup: avoid shadowing rb_ary_subseq
      pkg.mk: synchronize with my other projects
      gemspec: remove invalid rdoc_options
      gemspec: use SPDX license abbreviation