Make ScopedProcessRunner hold the child until we register with systemd

The QProcess::processStarted signal is too late: that is emitted after
the child process has execve'd the target executable. If that target
exits or backgrounds too quickly, the PID we get from processId() may be
stale. Hopefully it won't have got reused yet, but we get errors from
systemd nonetheless. And in the case of a target that forks and
backgrounds, the processes "escape" the confinement. This is seen with
Chrome/Chromium, which means their processes end up in
plasma-krunner.service and slice.

So, instead, we must hold the child process before execve() and we do
that with an eventfd. Once systemd has replied to us, we release the
child.

SystemdProcessRunner doesn't need this change.