* nfprobe: introduced support for variable-length custom-defined primitives
* fix, nfprobe: tackled the case of coexisting 1) PEN and non-PEN custom
  primitives and 2) variable and fixed custom primitives.
* fix, plugin_common.c: declared struct pkt_data in P_cache_insert_pending
  to be pointed by prim_ptrs. primptrs_set_all_from_chained_cache() is now
  safe if prim_ptrs is null (2)