in6_control: Serialize some SIOCs using netisr0

The serialized SIOCs are:
- IPv6 multicast routing.
- Source address selection policy (not used yet).
- Scope6.
- IPv6 address changes.
in6_control: Cosmetic cleanup
inet6: Simplify in6ifa_ifpwithaddr()
in6_mask2len: Constfy parameters