Fix exit with parent on macOS

It should be possible to do an [equivalent, although much trickier](https://gitlab.com/nbdkit/nbdkit/-/blob/master/common/utils/exit-with-parent.c#L84-159), variation of [what we currently do for Linux](https://github.com/appsignal/appsignal-wrap/blob/main/src/exit.rs) in macOS.