unicorn 6.1.0

This release reduces CPU usage for Linux 4.5+ in most cases.
See "[PATCH 6/6] use EPOLLEXCLUSIVE on Linux 4.5+" for more details:
  https://yhbt.net/unicorn-public/20211001030923.26705-7-bofh@yhbt.net/

There's a couple of updates for Ruby 3.1, but we've finally
started relying on Ruby 2.0.0 features after 9 years :P
(so Ruby 1.9.3 users are stuck with older versions).

And the usual round of doc updates and some build speedups.

13 changes by the Bozo Doofus maintainer since v6.0.0:

      test_util: less excessive encoding tests
      drop Ruby 1.9.3 support, require 2.0+ for now
      drop unnecessary IO#close_on_exec=true assignment
      extconf.rb: get rid of unnecessary checks
      makefile: reduce unnecessary rebuilds
      HACKING: drop outdated information about pandoc
      http_server: get rid of Process.ppid check
      worker_loop: get rid of select() avoidance hack
      use EPOLLEXCLUSIVE on Linux 4.5+
      allow Ruby to deduplicate remaining globals
      epollexclusive: remove rb_gc_force_recycle call
      drop Ruby version warning, fix speling errer
      doc: v3 .onion updates, nntp => nntps, minor wording changes