packet lib: ipv6: change some default parameters

nxt       : IPPROTO_NONE(0x3b) -> IPPROTO_TCP(0x06)
hop_limit :                  0 -> 255

it is more useful and common.

Signed-off-by: Yuichi Ito <ito.yuichi0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>