fix(process): safely exit forked child with _exit

[mmstick]

Replaces #1284

Though I cannot replicate it, this may fix some of the reports of the cosmic-app-library failing to open after spawning an application. And may fix the issue with NVIDIA GPUs having the validation error after spawning an application?

• Use waitpid to wait for the PID of the initial fork in the double fork to exit.
• Use libc::_exit in that initial fork after writing the PID of the double-forked child. This should ensure it exits without potentially affecting the state of the parent process.

After merging, this should be pulled in by cosmic-app-library, cosmic-files, and cosmic-launcher

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