[2.2] ProcessGroupLeaderMixin races with spawnProcess

ProcessGroupLeaderMixin.connectionMade tries to set the child process as a process group leader using setpgid. However, this is not permitted once the child process has called exec, which means the the parent process is racing with the child process, the former to call setpgid, the latter to call exec.

There are no synchronisation points in spawnProcess or hooks to customise the child's environment before exec is called, so ProcessGroupLeaderMixin needs to be removed, and the child processes need to set themselves as process group leaders.