networkd: introduce netdev ipvtap

This patch adds netdev ipvtap that is based on the
IP-VLAN network interface, called ipvtap. An ipvtap device can be created
in the same way as an ipvlan device, using 'kind ipvtap', and then accessed
using the tap user space interface.

test/fuzz/fuzz-netdev-parser/directives.netdev

@@ -151,6 +151,9 @@ VNetHeader=
 [IPVLAN]
 Mode=
 Flags=
+[IPVTAP]
+Mode=
+Flags=
 [Tun]
 OneQueue=
 MultiQueue=