[$] Rethinking bpfilter and user-mode helpers

The bpfilter subsystem, along with its"user-mode blobs" infrastructure, attracted a lot of attention when it wasmerged for the 4.18 kernel in 2018. Since then, however, development inthis effort has been, to put it charitably, subdued. Now, two years afterits merging, bpfilter may be in danger of being removed from the kernel asa failed experiment.