To help users check what version of libbpf is being used with bpftool,
print the number along with bpftool's own version number.

Output:

    $ ./bpftool version
    ./bpftool v5.16.0
    using libbpf v0.7
    features: libbfd, libbpf_strict, skeletons

    $ ./bpftool version --json --pretty
    {
        "version": "5.16.0",
        "libbpf_version": "0.7",
        "features": {
            "libbfd": true,
            "libbpf_strict": true,
            "skeletons": true
        }
    }

Note that libbpf does not expose its patch number.
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220210104237.11649-2-quentin@isovalent.com