tools: bpftool: reimplement bpf_prog_load() for prog load

bpf_prog_load() is a very useful helper but it doesn't give us full
flexibility of modifying the BPF objects before loading.  Open code
bpf_prog_load() in bpftool so we can add extra logic in following
commits.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>