initial fixes for building kj/main.c++ utilities on mingw

Note, this isn't all that's needed to fix the mingw build (cross compiling from linux).  In particular, this only fixes what I needed to use kj/main.c++ (and dependencies).

I'm assuming that the desired way of dealing with utf-16 windows APIs is to keep everything internally as utf-8 (as advocated by http://utf8everywhere.org/).

With wider windows API usage, we'll eventually want to extract a common way of doing the utf-8 <-> utf-16 translation, rather than calling MultiByteToWideChar all over the place.  I'm not sure what that will look like.
> I was thinking mostly of a lot of other APIs, where it does actually matter (file names, registry, UI, ...). 

Indeed. Luckily, KJ doesn't much interact with those APIs. E.g. we mostly expect the application code to open() files and then pass off the file descriptor to us, so the concept of file names is entirely sidestepped. :)

> I had looked into `WriteFileGather`, and it's a sad, lonely, largely useless API. It assumes that all the buffers are page-aligned and page-sized, so it doesn't really cover the writev use-case.

Oh right, I actually knew that at one time and forgot. Useless, I agree.