selftests/bpf: Expose and rename debug argument

Launching xdpxceiver with -D enables what was formerly know as 'debug'
mode. Rename this mode to 'dump-pkts' as it better describes the
behavior enabled by the option. New usage:

./xdpxceiver .. -D
or
./xdpxceiver .. --dump-pkts

Also make it possible to pass this flag to the app via the test_xsk.sh
shell script like so:

./test_xsk.sh -D

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210223162304.7450-3-ciara.loftus@intel.com