fix scan_local_interfaces()

when we delete an interface from all_ifaces, delete it from ifaces as well
remove its selected status if it was selected

at the moment, an interface that was used for capturing before will
never be removed from the list of interfaces even if it becomes
unavailable as it remains in ifaces and will be re-added to all_ifaces
in scan_local_interfaces()

new helper function capture_opts_del_iface() to delete an entry from ifaces and
free all its components

Change-Id: Ie3271a7ed086367e511d3a971f3b68cfc014115d
Reviewed-on: https://code.wireshark.org/review/2965
Reviewed-by: Evan Huus <eapache@gmail.com>