Learn about tmp directory from the TMPDIR environment variable

In the OS abstraction layer, we have this code:

```
char shm_path[] = "/dev/shm/soundio-XXXXXX";
char tmp_path[] = "/tmp/soundio-XXXXXX";
char *chosen_path;

int fd = mkstemp(shm_path);
if (fd < 0) {
fd = mkstemp(tmp_path);
if (fd < 0) {
return SoundIoErrorSystemResources;
} else {
chosen_path = tmp_path;
}
} else {
chosen_path = shm_path;
}
```

I think the part with `/dev/shm/*` is OK, and even the fallback to `/tmp/*` is OK. But let's also check TMPDIR. So it would be 1) try /dev/shm 2) try TMPDIR 3) try /tmp/*.

This would simplify part of #127