csv_to_fd: improve validation * Validate flags: - Ensure M and V can't occur more than once. - Ensure absence of V for vendor 0. - Ensure presence of V for vendor not 0. * Ensure AVP name is unique per vendor. * Add AVP name to most errors. * Use csv.DictReader.line_num for more accurate line numbers. TODO: enforce M and V must be present once and only once. Some AVPs currently fail that rule, including: - RFC 4005 / RFC 7155 QoS-Filter-Rule - 3GPP TS 29.212 PDN-Connection-ID - 3GPP TS 29.212 PS-to-CS-Session-Continuity