Rework handle_esp.c (proto, DPDK<17.08, cleanup)

Rework handle_esp.c (proto, DPDK<17.08, cleanup)

- Adjust handle functions to PROX-v035+ prototype, returning 0 when all
  packets have been handled.
- Because cryptodev API was modified in DPDK 17.08, many changes would
  be necessary to support it. For the moment, just exclude handle_esp.c
  from PROX build when using DPDK 17.08+, which will be supported later.
- Cleanup:
  - consistent use of PROX_PANIC to cleanly fail on errors
  - consistent use of OUT_DISCARD to drop unhandled packets
  - remove "__attribute__((unused))" when parameter is actually used
  - remove unused defines, fields and variables

Change-Id: I10d84fa184a866b4a5f87272875612aace5dd5b1
Signed-off-by: Patrice Buriez <patrice.buriez@intel.com>