uterm: video: use shl_dlist for linked lists

uterm: video: use shl_dlist for linked lists

uterm-video is actually older than shl_dlist and it used its own
single-linked lists until now. This had several drawbacks and produced
ugly code.

This commit converts all uterm-video elements to use shl_dlist and
introduces _bind and _unbind helpers.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>