tools: bpftool: fix bash completion for bpftool prog (attach|detach)

Fix bash completion for "bpftool prog (attach|detach) PROG TYPE MAP" so
that the list of indices proposed for MAP are map indices, and not PROG
indices. Also use variables for map and prog reference types ("id",
"pinned", and "tag" for programs).

Fixes: b7d3826c ("bpf: bpftool, add support for attaching programs to maps")
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>