packet: fix PathAttributeAsPath

PathAttributeAsPath can handle both two-bytes and four-bytes AS
number. Thus, PathAttributeAsPath serializer needs to know which way
should be used. This patch introduces As4PathParam. You can pass
either of As4PathParam and AsPathParam struct to initialize
PathAttributeAsPath struct.

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