[WIP] 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 should alleviate any concerns about `fwrite()` being slower than `write()`.