libbpf: improve requesting and getting BPF verifier logs

[anakryiko]

Extend libbpf APIs to allow more flexible way to request BPF verifier logs. Currently libbpf automatically will log BPF verifier logs on BTF load failure or BPF program load failure through its print callback. But there are many situations where getting verbose logs on success is important. Similarly, parsing out verification logs from print callback might be unnecessarily convoluted in some cases.

So extend libbpf APIs to allow more control over this.