Improve error messages from bpf_open(), and let it do the error handling.

Improve error messages from bpf_open(), and let it do the error handling.

bpf_open() already handles returning the right PCAP_ERROR_ value and
setting p->errbuf; let it do its thing.

Enhance its thing so that it tries to do a better job of figuring out
what the problem is (no BPF devices at all, all BPF devices busy, no
permission to open BPF device, something else).