libbpf_print_fn function to display CO-RE relocations debug information

[rafaeldtinoco]

I had already in mind that libbpf_print_fn could be used to display bpf object load debug messages from libbpf, specially for us to debug CO-RE rellocations issues. After the following comment from upstream:

aquasecurity/tracee#950 (comment)

we may also need to filter out some other errors (which I'll investigate in this issue altogether).

[geyslan]

This's being tackled, in part, by #284 (filtering callbacks). It's up to the consumers to define the filtering they want.