Merge branch 'bpftool-net-attach'

Daniel T. Lee says:

====================
Currently, bpftool net only supports dumping progs attached on the
interface. To attach XDP prog on interface, user must use other tool
(eg. iproute2). By this patch, with `bpftool net attach/detach`, user
can attach/detach XDP prog on interface.

    # bpftool prog
        16: xdp  name xdp_prog1  tag 539ec6ce11b52f98  gpl
        loaded_at 2019-08-07T08:30:17+0900  uid 0
        ...
        20: xdp  name xdp_fwd_prog  tag b9cb69f121e4a274  gpl
        loaded_at 2019-08-07T08:30:17+0900  uid 0

    # bpftool net attach xdpdrv id 16 dev enp6s0np0
    # bpftool net
    xdp:
        enp6s0np0(4) driver id 16

    # bpftool net attach xdpdrv id 20 dev enp6s0np0 overwrite
    # bpftool net
    xdp:
        enp6s0np0(4) driver id 20

    # bpftool net detach xdpdrv dev enp6s0np0
    # bpftool net
    xdp:

While this patch only contains support for XDP, through `net
attach/detach`, bpftool can further support other prog attach types.

XDP attach/detach tested on Mellanox ConnectX-4 and Netronome Agilio.

---
Changes in v5:
  - fix wrong error message, from errno to err with do_attach/detach

Changes in v4:
  - rename variable, attach/detach error message enhancement
  - bash-completion cleanup, doc update with brief description (attach
    types)

Changes in v3:
  - added 'overwrite' option for replacing previously attached XDP prog
  - command argument order has been changed ('ATTACH_TYPE' comes first)
  - add 'dev' keyword in front of <devname>
  - added bash-completion and documentation

Changes in v2:
  - command 'load/unload' changed to 'attach/detach' for the consistency
====================

Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>