Using MAINPID breaks systemd's exit detection, as it stops watching the
original pid, but is unable to watch the new pid as it is not a child
of systemd itself. The best we can do is just notify when execing the child
process.

We also need to consolidate forking into a sigle place so that we don't
end up with multiple levels of child processes if both redirecting log
output and reaping child processes.
Signed-off-by: Brad Davidson <brad.davidson@rancher.com>
(cherry picked from commit dc18ef2e)