Probe kernel bpf2bpf support

[ti-mo]

The adjustments performed by fixupAndValidate() don't rely on CO-RE being
applied first. Execute it earlier in the chain to potentially avoid loading
BTF and executing CO-RE when we could bail out instead.

Also avoids having to touch test cases due to changes in a subsequent commit.

When trying to load a program containing bpf2bpf calls on a kernel that
doesn't support it, we bubble up the verifier error to the caller:

  program xdp_prog: load program: invalid argument: unreachable insn 28

This is clear, but doesn't allow skipping tests when loading e.g. loader.c
on older kernels due to it containing bpf2bpf calls.

Implement a feature probe for bpf2bpf, return an unsupported error when
trying to load a program with multiple subprogs on an older kernel.

Update the test suite to skip unsupported features instead of hardcoding
a skip on 4.16 and earlier.

• Provide clear(er) and earlier error feedback about the kernel's bpf2bpf support
• Allow test skips when loading a bpf2bpf prog on a kernel that doesn't support it
• Remove hardcoded feature gates for bpf2bpf on kernel 4.16

[lmb]

Thanks, checking after loading is nicer!

[ti-mo]

@lmb I've found a bug introduced in 9b5a039 and made some changes to ErrorWithLog. PTAL 🙏

I've also added hasReferences() and looked at err instead of logErr.

[lmb]

Looks good, thanks for looking at ErrorWithLog as well