Co-Authored-By: lemoer <git@irrelefant.net>

Fixes respondd on 64bit archs, as gluonutil_get_primary_domain() was
assumed to return int without the prototype.

Fixes: bcf57467 ("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.

Implemented using readlink() and basename() to the selected domain in
/lib/gluon/domains/${DOMAIN}.json.

[Matthias Schiffer: rebase and simplify]

libgluonutil is not usable outside the OpenWrt/LEDE environment anyways, so
it doesn't make much sense to make the CMakeLists.txt overly generic.