shl: hook: add 'oneshot' flag

shl: hook: add 'oneshot' flag

If an entry is marked as 'oneshot' then it will get deleted after it has
been called once. Note that the entry is unlinked _before_ the callback is
called.

If you use 'oneshot'-entries and normal entries with the same cb+data
combination, then you will probably get unexpected behavior. It is not
recommended to do that. In detail, you cannot control which entry is
deleted via a shl_hook_rm() call so you can never be sure whether the
oneshot entry or a normal entry is deleted.

Do not mix oneshot entries with normal entries!

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