[WIP] remove HAVE_DUP

Vim defines this for Windows, so there's no Neovim-supported system for which this would not be defined.

Also obviates https://code.google.com/p/vim/source/detail?r=v7-4-216 .

MSDN says `dup()` is deprecated, so maybe we should have an `os_dup()` that wraps `_dup` for Windows.