get rid of struct carp_if by moving the srpl into struct ifnet if_carp.

get rid of struct carp_if by moving the srpl into struct ifnet if_carp.

currently carp uses a struct carp_if to hold an srp list head, which
is accessed by both if_carp in struct ifnet, and via the if input
handlers list.

this gets rid of some indirection by making if_carp itself the list
head, rather than a pointer to the list head via a struct carp_if.
it also makes accessing the list consistent by only using if_carp
to get to it.

ok mpi@