Put the extcap interfaces at the *end* of the interface list.

Put the extcap interfaces at the *end* of the interface list.

The list should start with the native local interfaces, as returned by
pcap, so that we default to the interface pcap gives first, rather than
to whatever extcap interface happens to be at the beginning.

This also means that, if we're only calling extcap_interface_list() to
regenerate our internal data structures, we don't bother allocating -
and leaking! - a list of if_info structures.

Change-Id: Ida651b5b081883f118a300b9f57403f2dc5c4363
Ping-Bug: 12183
Reviewed-on: https://code.wireshark.org/review/14187
Reviewed-by: Guy Harris <guy@alum.mit.edu>