[RFC] Non-unix-specific os_unix function. 

I noticed that `mch_write()` looked just a stone's throw from being OS-independent. Posting early to see if anyone can point out a flaw in my logic.

A few other functions in `os_unix.c` don't really depend on unix, like `mch_init()`, so I plan to add more to this.
This actually shouldn't be a problem, I once read an article benchmarking a lot of different options and when push comes to shove, it seems the C stdlib options aren't noticeably noticeably slower (on OSX), the buffering sometimes even helps. That said, sometimes it's counterproductive, depends on the use-case.

But you should test on your system. I found the testfile on my system but can't find the article again: https://gist.github.com/aktau/8f2a18ef4eb1e4f3c0a8