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: Fix login script to not use hardcoded debug values
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-desktop-guest-session: Syntax fixes
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.