bpf: use BPF_F_ALLOW_MULTI flag if it is available

bpf: use BPF_F_ALLOW_MULTI flag if it is available

This new kernel 4.15 flag permits that multiple BPF programs can be
executed for each packet processed: multiple per cgroup plus all
programs defined up the tree on all parent cgroups.

We can use this for two features:

1. Finally provide per-slice IP accounting (which was previously
   unavailable)

2. Permit delegation of BPF programs to services (i.e. leaf nodes).

This patch beefs up PID1's handling of BPF to enable both.

Note two special items to keep in mind:

a. Our inner-node BPF programs (i.e. the ones we attach to slices) do
   not enforce IP access lists, that's done exclsuively in the leaf-node
   BPF programs. That's a good thing, since that way rules in leaf nodes
   can cancel out rules further up (i.e. for example to implement a
   logic of "disallow everything except httpd.service"). Inner node BPF
   programs to accounting however if that's requested. This is
   beneficial for performance reasons: it means in order to provide
   per-slice IP accounting we don't have to add up all child unit's
   data.

b. When this code is run on pre-4.15 kernel (i.e. where
   BPF_F_ALLOW_MULTI is not available) we'll make IP acocunting on slice
   units unavailable (i.e. revert to behaviour from before this commit).
   For leaf nodes we'll fallback to non-ALLOW_MULTI mode however, which
   means that BPF delegation is not available there at all, if IP
   fw/acct is turned on for the unit. This is a change from earlier
   behaviour, where we use the BPF_F_ALLOW_OVERRIDE flag, so that our
   fw/acct would lose its effect as soon as delegation was turned on and
   some client made use of that. I think the new behaviour is the safer
   choice in this case, as silent bypassing of our fw rules is not
   possible anymore. And if people want proper delegation then the way
   out is a more modern kernel or turning off IP firewalling/acct for
   the unit algother.