bpftool: Support sendmsg{4,6} attach types

Add support for recently added BPF_CGROUP_UDP4_SENDMSG and
BPF_CGROUP_UDP6_SENDMSG attach types to bpftool, update documentation
and bash completion.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>