merge revision(s) 43373: [Backport #9036]

	* io.c (rb_io_reopen): create a new, temporary FD via rb_sysopen and
	  call rb_cloexec_dup2 on it to atomically replace the file fptr->fd
	  points to.  This leaves no possible window where fptr->fd is invalid
	  to userspace (even for any threads running w/o GVL).  based on the
	  patch by Eric Wong <normalperson@yhbt.net> at [ruby-core:57943].
	  [Bug #9036]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@47545 b2dd03c8-39d4-4d8f-98ff-823fe69b080e