packet: add New*Prefix functions

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
packet: add NewPathAttributer* functions

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
packet add NewBGP*Message functions

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
packet add NewCap* functions

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>