If one of the ppp peers sends a packet with an acknowledge flag,
the ppp payload will be empty and DecodePPP will return TM_ECODE_FAILED.
To handle this case, the packet_length field in the GRE extended header (https://tools.ietf.org/html/rfc2637#section-4.1) is used.
DecodeGRE no longer tries to parse PPP payload if packet_length is zero.
On MinGW the result of ntohl needs to be casted to uint32_t and
the result of ntohs to uint16_t. To avoid doing this everywhere
add SCNtohl and SCNtohs macros.
The Source Routing Header had routing defined as a char* for a field
of variable size. Since that field was not being used in the code, I
removed the pointer and added a comment.