update the tclass API implementation not to erase the ECN bits.  - do not override if a non-zero value is already set.  - check the diffserv field and the ecn field separately. as suggested in draft-itojun-ipv6-tclass-api-03.txt.
update the tclass API implementation not to erase the ECN bits.
 - do not override if a non-zero value is already set.
 - check the diffserv field and the ecn field separately.
as suggested in draft-itojun-ipv6-tclass-api-03.txt.

MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame