kgio 2.11.3

Some doc and warning fixes for newer Rubies.

We're no longer on bogomips.org since it's due
for expiry and I can't pay extortionists for a .org, so
s/bogomips.org/yhbt.net/ for now, and be prepared to move again
when extortionists move onto the .net TLD.

      pkg.mk: use --local to "gem install"
      pkg.mk: use dark216 theme for Earth Day 2019
      test: fix warnings with RUBYOPT=-w
      tests: fix unused variable warnings from newer Rubies
      test_connect_fd_leak: do not close socket if non-existent
      test_syssend: avoid warning on cleanup
      build: remove olddoc from the gemspec
      doc: remove private email and outdated gmane archives
      doc: move from bogomips.org/kgio to yhbt.net/kgio