networkd: refuse to use .network files with missing Address/Gateway key

These keys are mandatory in [Address]/[Route] sections. Otherwise, we
hit an assert:

ens3: setting addresses
Assertion 'address->family == 2 || address->family == 10' failed at /build/amd64-generic/tmp/portage/sys-apps/systemd-9999-r1/work/systemd-9999/src/network/networkd-address.c:137, function address_configure(). Aborting.

Reported-by: Alex Polvi <alex.polvi@coreos.com>
At the same time make sure Route's Destination and Gateway uses the same address family.