Do not modify route info sockaddr in rtm_xaddrs().

Do not modify route info sockaddr in rtm_xaddrs().

The rti_info array is used to describe routes that should be found
by lookup.  Modifying the addreses in it is not a good idea.  There
were places where rtm_xaddrs() tried to fix the address family
instead of validating it.  Replace the modification with a check
and error out with EAFNOSUPPORT on failure.  Route labels always
have AF_UNSPEC and the other types are not used anyway.

OK kn@