shared/bpf: install log callback and suppress most messages from libbpf

shared/bpf: install log callback and suppress most messages from libbpf

$ build/test-socket-bind
...
libbpf: load bpf program failed: Operation not permitted
libbpf: failed to load program 'sd_bind4'
libbpf: failed to load object 'socket_bind_bpf'
libbpf: failed to load BPF skeleton 'socket_bind_bpf': -1
Failed to load BPF object: Operation not permitted

Now all lines with "libbpf:" are at debug level and will be hidden by
default.

Partially fixes https://bugzilla.redhat.com/show_bug.cgi?id=2084955#c14
(i.e. the error that was exposed when the initial error was fixed.)