libbpf: Add subskeleton scaffolding

In symmetry with bpf_object__open_skeleton(),
bpf_object__open_subskeleton() performs the actual walking and linking
of maps, progs, and globals described by bpf_*_skeleton objects.

Signed-off-by: Delyan Kratunov <delyank@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/6942a46fbe20e7ebf970affcca307ba616985b15.1647473511.git.delyank@fb.com