app/testpmd: add a verbose mode to checksum forward engine

app/testpmd: add a verbose mode to checksum forward engine

If the user specifies 'set verbose 1' in testpmd command line,
the csum forward engine will dump some informations about received
and transmitted packets, especially which flags are set and what
values are assigned to l2_len, l3_len, l4_len and tso_segsz.

This can help someone implementing TSO or hardware checksum offload to
understand how to configure the mbufs.

Example of output for one packet:

 --------------
 rx: l2_len=14 ethertype=800 l3_len=20 l4_proto=6 l4_len=20
 tx: m->l2_len=14 m->l3_len=20 m->l4_len=20
 tx: m->tso_segsz=800
 tx: flags=PKT_TX_IP_CKSUM PKT_TX_TCP_SEG
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Signed-off-by: Olivier Matz <olivier.matz@6wind.com>