core/bpf: prefix log messages from different bpf subsystems

core/bpf: prefix log messages from different bpf subsystems

When something goes awry, we would get identical log messages from all the
bpf subsystems. E.g. "Failed to load BPF object: %m" appeared 5 times in the
sources. But it is very important to know *which* object we failed to load.
This could be guessed, e.g. from surroudning messages or from filename/line
metadata, but when we get log messages in bug reports, this might not be
available. Let's make the messages distinguishable.

While at it, some messages were adjusted a bit. In particular, we shouldn't use
internal names like BPFProgram which have no meaning outside of the codebase.