Input manager save fix

This is trying to fix https://github.com/CleverRaven/Cataclysm-DDA/pull/7283#issuecomment-40890137

It works fine for me on linux, so I guess it's a windows thing. The fact that no error pops up indicates to me it that the `rename` function at the end of `input_manager::save` fails (it's the only possible place that can fail without an exception). And apparently windows implementation of this function differs as it does not override existing files.

I added a wrapper in file_wrapper.cpp for this function and use this. Note for testing: the current code will succeed the time it is used (as there is no other file there that has to be overridden). As soon as you change the key bindings again, they would not be saved.

Note: this must be tested on windows.