packet lib: ipv6: remove unnecessary import

Signed-off-by: itoyuichi <ito.yuichi0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
packet lib: ipv6: support udp

Signed-off-by: itoyuichi <ito.yuichi0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
packet lib: ipv6: simplify the operations of __init__()

before applying this patch:
  - ipv6.parser() uses 'nxt' of the last extension header
  - ipv6.__init__() rewrites 'nxt' in conjunction with extension headers
  - 'nxt' of the extension headers are set automatically, i.e. they are obscure

after applying this patch:
  - ipv6.parser() does not consider 'nxt' of the extension headers
  - ipv6.__init__() does not rewrite 'nxt'
  - 'nxt' of the extension headers are set manually as an argument of __init__()

Signed-off-by: itoyuichi <ito.yuichi0@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
packet lib: ipv6: avoid dangerous default value [] as argument

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