*: convert `struct interface->connected` to DLIST

*: convert `struct interface->connected` to DLIST

Replace `struct list *` with `DLIST(if_connected, ...)`.

NB: while converting this, I found multiple places using connected
prefixes assuming they were IPv4 without checking:

- vrrpd/vrrp.c: vrrp_socket()
- zebra/irdp_interface.c: irdp_get_prefix(), irdp_if_start(),
  irdp_advert_off()

(these fixes are really hard to split off into separate commits as that
would require going back and reapplying the change but with the old list
handling)

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>