libgluonutil: introduce gluonutil_get_interface_lower() and gluonutil_get_interface_type()

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.