Introduce probes for supported BPF program types in libbpf, and call it
from bpftool to test what types are available on the system. The probe
simply consists in loading a very basic program of that type and see if
the verifier complains or not.

Sample output:

    # bpftool feature probe kernel
    ...
    Scanning eBPF program types...
    eBPF program_type socket_filter is available
    eBPF program_type kprobe is available
    eBPF program_type sched_cls is available
    ...

    # bpftool --json --pretty feature probe kernel
    {
        ...
        "program_types": {
            "have_socket_filter_prog_type": true,
            "have_kprobe_prog_type": true,
            "have_sched_cls_prog_type": true,
            ...
        }
    }

v5:
- In libbpf.map, move global symbol to a new LIBBPF_0.0.2 section.
- Rename (non-API function) prog_load() as probe_load().

v3:
- Get kernel version for checking kprobes availability from libbpf
  instead of from bpftool. Do not pass kernel_version as an argument
  when calling libbpf probes.
- Use a switch with all enum values for setting specific program
  parameters just before probing, so that gcc complains at compile time
  (-Wswitch-enum) if new prog types were added to the kernel but libbpf
  was not updated.
- Add a comment in libbpf.h about setrlimit() usage to allow many
  consecutive probe attempts.

v2:
- Move probes from bpftool to libbpf.
- Remove C-style macros output from this patch.
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>