[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.
The issue here is that when I didn't add the `fflush()` line (not needed with `setbuf(stdout, NULL)`), I didn't see anything on screen, which means we can't let the data from `term_write()` sit in the buffer. So we either need to immediately flush after every write, or disable buffering.