resolv.rb: close socket

	* lib/resolv.rb (UnconnectedUDP#lazy_initialize): store new
	  sockets before binding, so the sockets get closed when the
	  requester is closing.

	* lib/resolv.rb (ConnectedUDP#lazy_initialize): ditto.

	* lib/resolv.rb (UnconnectedUDP#close): synchronize to get rid of
	  race condition.

	* lib/resolv.rb (ConnectedUDP#close): ditto.

	[ruby-core:85901] [Bug #14571]

	From: quixoten (Devin Christensen) <quixoten@gmail.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@62915 b2dd03c8-39d4-4d8f-98ff-823fe69b080e