Implement a POSIX command runner by using fork/exec.

This implementation is optimized to run commands in the background, and has a few noteworthy implementation details.

First off, when the child process is forked, it calls setsid() to detach from the controlling TTY. This has two main ramifications: sending signals will never be send to the forked process, and the forked process does not have access to stdin.

@api private