Fix crash on double pthread_cancel(3) on exit

Fix crash on double pthread_cancel(3) on exit

On OpenBSD 7.4-current, failure to listen on the RTSP socket(s) results
the `rtsp_listener_thread` being pthread_cancel(3)'ed twice, once through
`rtsp_listen_loop()` and again via atexit(3) handler `exit_rtsp_listener()`:

```
$ nc -4l 5000 &
$ nc -6l 5000 &
$ shairport-sync -c/dev/null
warning: could not establish a service on port 5000 -- program terminating. Is another instance of Shairport Sync running on this device?
Segmentation fault (core dumped)
```
```
Program terminated with signal SIGSEGV, Segmentation fault.
433             if (tib->tib_canceled == 0 && tid != 0 &&
[Current thread is 1 (process 290061)]
```

`die()` -> `exit(EXIT_FAILURE)` normally in this case, thus forgoing the
first cancel and relying on the atexit handler alone.

With Mike Brady.