tools: bpftool: mark offloaded programs more explicitly in plain output

In bpftool (plain) output for "bpftool prog show" or "bpftool map show",
an offloaded BPF object is simply denoted with "dev ifname", which is
not really explicit. Change it with something that clearly shows the
program is offloaded.

While at it also add an additional space, as done between other
information fields.

Example output, before:

    # bpftool prog show
    1337: xdp  tag a04f5eef06a7f555 dev foo
            loaded_at 2018-10-19T16:40:36+0100  uid 0
            xlated 16B  not jited  memlock 4096B

After:

    # bpftool prog show
    1337: xdp  tag a04f5eef06a7f555  offloaded_to foo
            loaded_at 2018-10-19T16:40:36+0100  uid 0
            xlated 16B  not jited  memlock 4096B

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>