Add systemd support

Now that Debian/Ubuntu are switching to systemd as default, it's time to Epoptes provide a better systemd support, which envolves:

 * Ship systemd service unit files for both epoptes[-server] and epoptes-client.

 * Add "systemctl reboot" and "systemctl poweroff" fallbacks to /usr/share/epoptes-client/endsession.

 * Gather more information from logind, specially about seats.

About service unit files, I've written the following examples for openSUSE package:

epoptes.service:

[Unit]
Description=A twisted-based daemon that manages epoptes-client
After=syslog.target

[Service]
Type=forking
PIDFile=/run/epoptes.pid
ExecStartPre=/bin/sh -c "if ! [ -s /etc/epoptes/server.key ] || ! [ -s /etc/epoptes/server.crt ]; then openssl req -batch -x509 -nodes -newkey rsa:1024 -days 3652 -keyout /etc/epoptes/server.key -out /etc/epoptes/server.crt; chmod 600 /etc/epoptes/server.key; fi"
ExecStart=/usr/bin/twistd --pidfile /var/run/epoptes.pid --logfile /var/log/epoptes.log epoptes

[Install]
WantedBy=multi-user.target

epoptes-client.service:

[Unit]
Description=Epoptes-client system daemon
Wants=network-online.target
After=network-online.target

[Service]
EnvironmentFile=-/etc/default/epoptes-client
ExecStartPre=/bin/sh -c "if ! [ -s /etc/epoptes/server.crt ]; then /usr/sbin/epoptes-client -c ${SERVER}; fi"
ExecStart=/usr/sbin/epoptes-client

[Install]
WantedBy=multi-user.target

NOTE: network-online.target may require that you explicitly enable the *-wait-online.service that matches your current network management mechanism (NetworkManager, systemd-networkd, etc.)

In the case one requires that epoptes-client service automatically restarts if connection is lost, he can provide the following drop-in:

/etc/systemd/system/epoptes-client.service.d/auto-reconnect.conf:

[Service]
Restart=always