libbpf: add compat helpers for libbpf down to 0.1.0

libbpf: add compat helpers for libbpf down to 0.1.0

- new symbols are available from libbpf 0.6.0 so could be used with
libbpf.so.0, but we're sure the old symbols will be there and this
simplifies code
- detection at runtime should always work, regardless of whether systemd
has been compiled with older or newer libbpf and runs with older or newer
libbpf