shl: hook: add helpers to add callbacks only once

shl: hook: add helpers to add callbacks only once

Very often we want to avoid adding callbacks multiple times. Currently we
have to keep track of this in each module with a boolean value.
To simplify this procedure, we add helpers that guarantee that entries are
only added once and a helper to remove all entries with a given cb+data
combination.

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