systemd: make NetworkManager reloadable via SIGHUP

systemd: make NetworkManager reloadable via SIGHUP

Since f9e4af2, parts of the configuration can be reloaded
by sending SIGHUP to NetworkManager. Add ExecReload option
to service file to support reloading by sending a signal.

Note that 'man 5 systemd.service' advices to use a blocking
command instead of a sending a signal. Later we should add a
D-Bus method to allow reloading synchronously. For now, this
is better then nothing.

https://mail.gnome.org/archives/networkmanager-list/2015-April/msg00042.html