Instead of rewriting error code returned by the kernel of prog load with
libbpf-sepcific variants pass through the original error.

There is now also no need to have a backup generic -LIBBPF_ERRNO__LOAD
fallback error as bpf_prog_load() guarantees that errno will be properly
set no matter what.

Also drop a completely outdated and pretty useless BPF_PROG_TYPE_KPROBE
guess logic. It's not necessary and neither it's helpful in modern BPF
applications.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211209193840.1248570-7-andrii@kernel.org