lib/packet: make protocol_name property instead of attributes

Because it is read-only and to prevent accidental over-write.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
lib/packet: make packet_base.parser abstract method

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
lib/packet: teach packet library to truncate padding octet

The patch teaches packet library to truncate padding octets.
Change packet_base.parser() to return (header, next_type, rest_of_packet)
The protocol class that knows its payload length should rest_of_packet
where padding octets at the last of packet is truncated.

As bonus,
- fix ipv6 parser as ipv6 header doesn't have options.
  It seems copy-and-paste from ipv4
- improve ipv4, tcp a bit

Cc: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Cc: Shaun Crampton <Shaun.Crampton@metaswitch.com>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>