[RFC] vim-patch:7.4.456

```
Problem:  'backupcopy' is global, cannot write only some
          files in a different way.
Solution: Make 'backupcopy' global-local. (Christian Brabandt)

https://code.google.com/p/vim/source/detail?r=v7-4-456
```

Succeeds #1228. Changed the type for the flags per https://github.com/neovim/neovim/pull/1228#discussion_r18125563, but used `unsigned int` instead of suggested `int` (seems to be used for other flags, e.g. https://github.com/neovim/neovim/blob/7fc7f026ad1103b1f6670507cd9cd7a816ae3539/src/nvim/option_defs.h#L310).