bpf: beef up bpf detection, check if BPF_F_ALLOW_MULTI is supported

bpf: beef up bpf detection, check if BPF_F_ALLOW_MULTI is supported

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.