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.