misc: Add sys_slist_insert

sys_slist_insert can be used to insert a node in the middle of the list
which previously was not possible with sys_slist_append and
sys_slist_prepend.

Change-Id: Ib9e319469cd4911adf7ddf49c54c3f7390c4c953
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>