Fixes respondd on 64bit archs, as gluonutil_get_primary_domain() was
assumed to return int without the prototype.
Fixes: bcf57467dd ("libgluonutil: implement gluonutil_get_primary_domain()")
gluonutil_get_interface_lower() recursively determines the lower of an
interface until the base interface is found.
gluonutil_get_interface_type() tries do distinguish wired, wireless and
tunnel interfaces. For tunnel interfaces, TUN/TAP, L2TPETH and Wireguard
are supported at the moment.
