This improves the BPF/cgroup detection logic, and looks whether
BPF_ALLOW_MULTI is supported. This flag allows execution of multiple
BPF filters in a recursive fashion for a whole cgroup tree. It enables
us to properly report IP accounting for slice units, as well as
delegation of BPF support to units without breaking our own IP
accounting.
This path to ping is compatible with both debian-like and usr-merged
distros. This keeps the test simple, and should thus pass everywhere.
Fixes: #7267
