v1.16.0

yahns 1.16.0

This release fixes warnings for users combining Rack::Deflater
with HTTPS support:
    https://yhbt.net/yahns-public/20180714005630.11812-1-e@80x24.org/

yahns-rackup(1) users may not specify "-O listen=inherit"
when spawning from systemd.  The "listen" directive may
be omitted entirely from the yahns-config(5) files.

A few other cleanups and nothing interesting, otherwise.

20 yawn-worthy changes since v1.15.0 (2017-03-23):
      config: more descriptive variable name
      proxy_pass: comment explaining what rack.hijack calls
      USR2 upgrades may use Process.spawn for vfork
      avoid Thread#[] and Thread#[]= across threads
      gemspec: declare Ruby 2.0+ dependency, here
      test_bin: SO_KEEPALIVE value only needs to be true
      rackup_handler: remove unnecessary branch
      test_bin: use RbConfig.ruby for non-standard names
      allow omitting specifying socket name when inheriting listen socket
      server: fix incomplete comment about SSLContext#setup
      test_rack_env: additional test for Rack environment
      test: allow setting TAIL env to watch error logs
      fix some unused variables
      tests: thread-safety fixes
      test/server_helper: describe reason for termination
      test/test_ssl: set SSLContext#security_level=0
      doc: https:// URLs instead of git://
      http_client: clear backtrace on "wrong version number" in OpenSSL
      openssl_client: do not attempt writes after SystemCallError
      use IO#pread if available in Ruby 2.5

*ZZZZZZ*