libweston: strdup head make, model, serial_number

Duplicate these strings to decouple their lifetime from whatever the
backends used. This should prevent hard to catch use after frees and
such problems in the future.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
v5 Reviewed-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Ian Ray <ian.ray@ge.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Derek Foreman <derekf@osg.samsung.com>