Dear Maintainer,

When I shut down the computer, nodm does not quit when told to, but just
restarts my session, leading to a minor but annoying delay until systemd
times out and forcibly terminates it.

It seems that this is caused by systemd killing my session and stopping
nodm in parallel. I can easily reproduce the problem by doing:

pkill -u ryan nodm & pkill -u root nodm

I guess this is caused by nodm only listening for the quit signal while
it's in its main loop, but not while it's busy managing children.

It would be nice if nodm would respect the stop command and abort the
session restart.