tlist: Add tlist_next() and tlist_prev() functions

An odd omission from the tlist module is basic tlist_next() and
tlist_prev() macros matching list_next() and list_prev() in the basic
list module.  This adds them.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>