server: fix daemon mode on macOS with libao

[mohd-akram]

libao calls into macOS frameworks which do not behave well if exec is not called after fork.

Fixes #907

[sthibaul]

I reworked things a bit, and notably split the changes into separate commits, to much clearly show what is happening here, so further bisecting etc. will be much easier.

Thanks!

[mohd-akram]

Thanks! Regarding the modules, is there a reason for loading them before the fork? I noticed speechd waits a bit before going into daemon mode, and moving the module loading after resolves that. It also makes the modules children of the daemon process rather than not.

[sthibaul]

Regarding the modules, is there a reason for loading them before the fork?

It allows to see the failure to load any module at all, before speechd goes to background and is not to be seen again :)