client: Add ltspguestsshd configuration files for thin client guest session support
client: Actual thin client guest session support. Creates automatically
guest-<hostname> user both on the thin client and on the ltspserver.
(inetd listener is added to TCP 908 on the ltspserver to create the users.)
Lightdm guest account scripts are overwriten on thinclients.

As lightdm's guest session support does not run through PAM stack,
thin client login is done in xsession script. Also normal logins need
to be changed to same model.
client: Fix thinclient guest session cleanup when session finishes.
xexit scripts need LTSP_CLIENT_HOSTNAME set to work properly.
client: Create home directory for thinclient guest users when
logging in through ltspguestsshd and remove the directory if it
already exists.
client: Use device specific language setting from device.json as a fallback
if there's no language set in puavo session.
client: Add a new Xsession.d script that sets PUAVO_SESSION_PATH if it is
not set, but /var/lib/puavo-desktop/users/<user>/puavo_session.json exists.
This is used now for guest sessions that cannot use the PAM scripts to
create the file and set env variables.
client/puavo-thin-guest-account: Create puavo session dir for guest
users and fetch session from puavo-rest automatically.
client: Use the new session starter script for all thin and fat clients.
client: Pass PUAVO_SESSION_ID from thinclient to ltsp server's inetd
listener instead of hostname when initialising the guest user before
ssh login. Still needs work to make user home directory cleaning more
robust.
client: Try harder to find the interface for default route.
client: Fetch device.json also for ltsp servers that need a separate
puavo-rest resource.
client: Use user > session > device fallbacks for locale setting.
bootserver: add script for listing images and their various "states"

The name of the script is prefixed with underscore to denote that its
currently experimental, it should not be trusted and the format of the
output can change.
client: use zram in laptops and fatclients
client: nvidia package has been renamed: nvidia-current => nvidia-304
client: configure pulseaudio default-{sink,source} from Puavo
client: let fatclients mount /tmp from local disk

Normally, the usage of /tmp is quite low, but it might experience
temporal usage spikes, which might cause troubles for low-mem fatclients
because their /tmp is attached to tmpfs. Mounting /tmp to local disk
helps in these situations.