[$] Generic iterators for BPF

BPF programs destined to be loaded into the kernel are generally written inC but, increasingly, the environment in which those programs run differssignificantly from the C environment. The BPF virtual machine andassociated verifier make a growing set of checks in an attempt to make BPFcode safe to run. The proposed addition of an iterator mechanism to BPFhighlights the kind of features that are being added - as well as theconstraints placed on programmers by BPF.