Change the Prefix field of the Route Information Option to be variable-length.

[y-kzm]

This PR enables prefixes to be sent in smaller chunks within the Route Information Options.

According to RFC 4191, Section 2.3: The Length field is 1, 2, or 3 depending on the Prefix Length. If Prefix Length is greater than 64, then Length must be 3. If Prefix Length is greater than 0, then Length must be 2 or 3. If Prefix Length is zero, then Length must be 1, 2, or 3.

• Before change

        route info option (24), length 24 (3):  2001:db8:1::1/128, pref=medium, lifetime=300s
             0x0000:  8000 0000 012c 2001 0db8 0001 0000 0000
             0x0010:  0000 0000 0001
           route info option (24), length 24 (3):  2001:db8:2::/64, pref=medium, lifetime=300s
             0x0000:  4000 0000 012c 2001 0db8 0002 0000 0000
             0x0010:  0000 0000 0000

• After change

        route info option (24), length 24 (3):  2001:db8:1::1/128, pref=medium, lifetime=300s
             0x0000:  8000 0000 012c 2001 0db8 0001 0000 0000
             0x0010:  0000 0000 0001
           route info option (24), length 16 (2):  2001:db8:2::/64, pref=medium, lifetime=300s
             0x0000:  4000 0000 012c 2001 0db8 0002 0000

[Neustradamus]

@y-kzm: Have you seen @robbat2 review?