networkd: use VRFs routing table for DHCP routes

networkd: use VRFs routing table for DHCP routes

When an interface has been enslaved to a VRF the received routes should
be added to the VRFs RT instead of the main table.

This change modifies the default behaviour of routes in the case where a
network belongs to an VRF.  When the user does not configure a
`DHCP.RouteTable` in a `systemd.network` file and the interface belongs
to a VRF, the VRFs routing table is used instead of RT_TABLE_MAIN.

When the user has configured a custom routing table for DHCP the VRFs
table is ignored and the users preference takes precedence.