bpftool: add pinmaps argument to the load/loadall

This new additional argument lets users pin all maps from the object at
specified path.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>