list: list_add_after and list_add_before functions

These make it easy to add a new element before or after an
existing element in the middle of the list.

The existing list_add and list_add_tail functions are trivially
reimplemented on top of these new functions.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>