app/testpmd: rework checksum forward engine

app/testpmd: rework checksum forward engine

The csum forward engine was becoming too complex to be used and
extended (the next commits want to add the support of TSO):

- no explaination about what the code does
- code is not factorized, lots of code duplicated, especially between
  ipv4/ipv6
- user command line api: use of bitmasks that need to be calculated by
  the user
- the user flags don't have the same semantic:
  - for legacy IP/UDP/TCP/SCTP, it selects software or hardware checksum
  - for other (vxlan), it selects between hardware checksum or no
    checksum
- the code relies too much on flags set by the driver without software
  alternative (ex: PKT_RX_TUNNEL_IPV4_HDR). It is nice to be able to
  compare a software implementation with the hardware offload.

This commit tries to fix these issues, and provide a simple definition
of what is done by the forward engine:

 * Receive a burst of packets, and for supported packet types:
 *  - modify the IPs
 *  - reprocess the checksum in SW or HW, depending on testpmd command line
 *    configuration
 * Then packets are transmitted on the output port.
 *
 * Supported packets are:
 *   Ether / (vlan) / IP|IP6 / UDP|TCP|SCTP .
 *   Ether / (vlan) / IP|IP6 / UDP / VxLAN / Ether / IP|IP6 / UDP|TCP|SCTP
 *
 * The network parser supposes that the packet is contiguous, which may
 * not be the case in real life.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>