Consolidate ifqueue macros for the upcoming ALTQ work.

Consolidate ifqueue macros for the upcoming ALTQ work.
We don't use ifqueue locking and don't intend to add it,
so the whole IF_LOCK / IF_UNLOCK compatibility with
FreeBSD 5 can just be removed. Similiar the "unlocked"
versions of IF_PREPEND, IF_DEQUEUE and IF_ENQUEUE
(with leading _) are replaced with the normal versions.
IF_QEMPTY from the i4b sources is added to iv_var.h for
symmetry with the ALTQ macros, but not yet used elsewhere.