tools/bpftool: Add name mappings for SK_LOOKUP prog and attach type

Make bpftool show human-friendly identifiers for newly introduced program
and attach type, BPF_PROG_TYPE_SK_LOOKUP and BPF_SK_LOOKUP, respectively.

Also, add the new prog type bash-completion, man page and help message.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200717103536.397595-14-jakub@cloudflare.com