iface_lists: Access ifaces member by reference

iface_lists: Access ifaces member by reference

Change access of ifaces elements from by val to by reference.
With this change unnecessary copying of the whole struct is avoided
but even more important is that elements no longer have to be
removed and inserted whenever data is updated.

This change aims to make it more clear that ifaces elements shall
never directly be removed from the array. Instead use function
capture_opts_del_iface

NOTE: Code for GTK UI not updated

Ping-Bug: 13864
Change-Id: I04b65d5ee36526b30d959b8e5a2a48a3c7c4f15b
Reviewed-on: https://code.wireshark.org/review/23204
Reviewed-by: Anders Broman <a.broman58@gmail.com>