OpenSSL::SSL::SSLSocket#accept may block indefinitely on clients
which negotiate the TCP connection, but fail (or are slow) to
negotiate the subsequent TLS handshake.  This prevents the
multi-threaded WEBrick server from accepting other connections.

Since the TLS handshake (via OpenSSL::SSL::SSLSocket#accept)
consists of normal read/write traffic over TCP, handle it in the
per-client thread, instead.

Furthermore, using non-blocking accept() is useful for non-TLS
sockets anyways because spurious wakeups are possible from
select(2).

* lib/webrick/server.rb (accept_client): use TCPServer#accept_nonblock
  and remove OpenSSL::SSL::SSLSocket#accept call
* lib/webrick/server.rb (start_thread): call OpenSSL::SSL::SSLSocket#accept
* test/webrick/test_ssl_server.rb (test_slow_connect): new test
  [ruby-core:83221] [Bug #14005]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60172 b2dd03c8-39d4-4d8f-98ff-823fe69b080e