(Fnetwork_interface_list, Fnetwork_interface_info):

(Fnetwork_interface_list, Fnetwork_interface_info):
Require HAVE_NET_IF_H and HAVE_SYS_IOCTL_H to include these fns.
(Fnetwork_interface_info): Check that ifreq struct has required
fields before accessing them; this requires that those fields are
defined as macros, which may be too restrictive on some platforms,
but it is better than failing on other platforms.
(syms_of_process): Only defsubr above fns when included.