bpf: unify main prog and subprog

Currently, verifier treat main prog and subprog differently. All subprogs
detected are kept in env->subprog_starts while main prog is not kept there.
Instead, main prog is implicitly defined as the prog start at 0.

There is actually no difference between main prog and subprog, it is better
to unify them, and register all progs detected into env->subprog_starts.

This could also help simplifying some code logic.

Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>