systemd: set KillMode=process

systemd: set KillMode=process

By default, when shutting NM down, systemd will kill everything in its
cgroup. But this can cause problems (eg, NM thinking that dhclient
crashed and then taking down an interface that it would otherwise have
left up). Fix this by setting KillMode=process, which tells systemd to
only kill NM itself, and let NM kill its children.

https://bugzilla.redhat.com/show_bug.cgi?id=876218