app/testpmd: add ability to split outgoing packets

app/testpmd: add ability to split outgoing packets

For CSUM forwarding mode add ability to copy & split outgoing packet
into the new mbuf that consists of multiple segments.
For TXONLY and CSUM forwarding modes add ability to make number of
segments in the outgoing packet to vary on a per packet basis.
Number of segments and size of each segment is controlled by
'set txpkts' command.
Split policy is controlled by 'set txsplit' command.
Possible values are: on | off | rand.
Tha allows to increase test coverage for TX PMD codepaths.

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>