win32.c: rb_w32_dup2

* win32/win32.c (rb_w32_dup2): extract from rb_cloexec_dup2() and
  redirect_dup2().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43890 b2dd03c8-39d4-4d8f-98ff-823fe69b080e