Replace extcap_interface_list() with append_extcap_interface_list().

Replace extcap_interface_list() with append_extcap_interface_list().

Pull the "rebuild the list of extcap interfaces and, optionally, return
a list of if_infos for them" into a separate
extcap_reload_interface_list() routine, call it in the cases where we
don't want the if_infos list, and have append_extcap_interface_list()
call it, asking it for the if_infos list, and then append the interfaces
to that list.

Change-Id: I07478ab133859484b3e0916144971639f961224b
Reviewed-on: https://code.wireshark.org/review/14189
Reviewed-by: Guy Harris <guy@alum.mit.edu>