Add new FOREACH_FROM variants of the queue(3) FOREACH macros which can
optionally start the traversal from a previously found element by
passing the element in as "var". Passing a NULL "var" retains the same
semantics as the regular FOREACH macros.
Kudos to phk for suggesting the "FROM" suffix instead of my original
proposal.
Reviewed by: jhb (previous version), rpaulo
MFC after: 1 week
a36e348a9f