datapath: Add 'usage' function using DPDK API

The usage function is only shown when the EAL args parse successfully.
Use the DPDK API to show this usage message always. This requires
making the function "public".

Change-Id: Id69ed152e065329f8060fcad0b5b71a3da855540
Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-by: Kevin Traynor <kevin.traynor@intel.com>
Reviewed-by: Mark B. Kavanagh <mark.b.kavanagh@intel.com>
datapath: Remove 'secs' from 'usage' message

The 'stats_int' parameter defines the interval to be used between
stat displays. Previously this could be measure in seconds. However,
with the introduction of multi-core support this is no longer the
case. This makes the usage message misleading, therefore remove
'secs'.

Change-Id: I28b387c2aa3be466ce800481e4d2da6b0cbbc22f
Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-by: Kevin Traynor <kevin.traynor@intel.com>
Reviewed-by: Mark B. Kavanagh <mark.b.kavnagh@intel.com>