launcher-logind: find viable session more agressively

Do not rely solely on sd_pid_get_session but also use sd_uid_get_sessions to find a viable systemd-logind session to use. This allows to run Weston from a ssh session or from within a container where systemd-logind API is not able to determine the active session. In both cases Weston will pick the active systemd session running on the seat Weston is using. In practise this means a free/non-graphical VT needs to be active when running Weston.

Fixes: #401 (closed) Fixes: #132 (closed) Signed-off-by: Stefan Agner stefan@agner.ch