mkstemp() in soundio_os_init_mirrored_memory() needs restrictive umask set

On POSIX systems, `soundio_os_init_mirrored_memory()` calls `mkstemp()` to create a temporary file as a basis for a later `mmap()` call. However, the `mkstemp.3` man page states

```
The old behavior of creating a file with mode 0666 may be a security
risk, especially since other Unix flavors use 0600, and somebody might
overlook this detail when porting programs.

More generally, the POSIX specification of mkstemp() does not say any-
thing about file modes, so the application should make sure its file
mode creation mask (see umask(2)) is set appropriately before calling
mkstemp() (and mkostemp()).
```

This can be addressed by placing something like

```
mode_t old_umask = umask(077);
```

before the call(s), and

```
umask(old_umask);
```

afterward.

(Alternately, might I suggest `tmpfile()` together with `fileno()`? That might be less hassle...)