MINOR: lists: add a LIST_DEL_INIT() macro

MINOR: lists: add a LIST_DEL_INIT() macro

It turns out that we call LIST_DEL+LIST_INIT very frequently and that
the compiler doesn't know what pointers get modified in the e->n->p
and e->p->n dance, so when LIST_INIT() is called, it reloads these
pointers, which is quite a bit of a mess in terms of performance.

This patch adds LIST_DEL_INIT() to perform the two operations at once
using local temporary variables so that the compiler knows these
pointers are left unaffected.