Add TLSv1.3 support

If OpenSSL supports TLSv1.3, enable it by default for all applications
under src.  BoringSSL can work at the moment although it does not
unlock all the features nghttpx offers.  OpenSSL's TLSv1.3 support is
still WIP at the time of writing.

nghttp2_session: fix The 'then' statement is equivalent to the subseq…

Add nghttp2_option_no_closed_streams

nghttp2_session: fix The 'then' statement is equivalent to the subsequent code fragment found by PVS Studio (V523)

This commit deprecates --tls-proto-list option, and adds 2 new
options: --tls-min-proto-version and --tls-max-proto-version to
specify minimum and maximum protocol version respectively.  Versions
between the two are enabled.  The deprecated --tls-proto-list has
empty default value, and acts like enabling only specific protocol
versions in the range for now.

nghttp2_option_no_closed_streams controls whether closed streams are
retained or not.  If nonzero is passed to that function's parameter
val, a session does not retain closed streams.  It may hurt the shape
of priority tree, but can save memory.

Previously, after sending SIGUSR2 to the original master process, and
the new master process gets ready, user has to send SIGQUIT to the
original master process to shut it down gracefully.  With this commit,
the new master process sends SIGQUIT to the original master process
when it is ready to serve requests, eliminating for user to send
SIGQUIT manually.

This works nicely with systemd, because now you can replace nghttpx
binary with new one by "systemctl kill -s USR2 --kill-who=main
nghttpx".

This reverts commit f9946649.

graceful stop of nghttp2::asio_http2::server::http2

nghttpx: add systemd support

Explicit io_service::stop() will prevent running streams from
finishing their task. That means if there are already reposnes
that we have called end(std::string) on them and they have not
finished sending back their data, they will be closed with a
NGHTTP2_INTERNAL_ERROR
Instead, we can stop accepting connections and destroy all
io_service::work objects to signals end of work.

  Add systemd's Type=notify support by sending information about
 master process PID around forks.
  Add some hardening option to service unit.