cpc_gre.h => fixed flexible occurence of CSUM, KEY, SEQNO
added checksum calculation for GRE to file packet_proto_meta_imp.h
cpc_gre.h => fixed bug in checksum calculations => do not construct an IP pseudo header, as it is not used in the GRE checksum calculation
cpc_gre.h => renamed constant GRE_PROT_TYPE_TRANSPARENT_ETHERNET_BRIDGING