libbpf: Add bpf_object__unpin()

For bpf_object__pin_programs() there is bpf_object__unpin_programs().
Likewise bpf_object__unpin_maps() for bpf_object__pin_maps().

But no bpf_object__unpin() for bpf_object__pin(). Adding the former adds
symmetry to the API.

It's also convenient for cleanup in application code. It's an API I
would've used if it was available for a repro I was writing earlier.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/bpf/b2f9d41da4a350281a0b53a804d11b68327e14e5.1692832478.git.dxu@dxuuu.xyz