networkd: DHCP client - ignore default route if classless static rout…

[ssahani]

…e is set.

According to RFC 3442:

If the DHCP server returns both a Classless Static Routes option and
a Router option, the DHCP client MUST ignore the Router option.

This work fixes #5695.

[poettering]

looks OK, but needs rebase.

[asavah]

This crap kills default route v4 altogether.
Was this even runtime tested?

Capture dhcp reply from the router:

          Your-IP 192.168.X.179
          Server-IP 192.168.X.1
          Client-Ethernet-Address X
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: ACK
            Server-ID Option 54, length 4: 192.168.X.1
            Lease-Time Option 51, length 4: 86400
            RN Option 58, length 4: 43200
            RB Option 59, length 4: 75600
            Subnet-Mask Option 1, length 4: 255.255.255.0
            BR Option 28, length 4: 192.168.X.255
            Hostname Option 12, length 4: "rpi3"
            Domain-Name Option 15, length 8: "X.Y"
            Domain-Name-Server Option 6, length 8: 192.168.X.1,192.168.X.5
            Default-Gateway Option 3, length 4: 192.168.X.1

DHCP server is dnsmasq.

No classless static routes, only gw.
On the client:

ip ro sh
192.168.X.0/24 dev eth0 proto kernel scope link src 192.168.X.179

[ssahani]

yikes messed up in the rebase. Please try with ssahani@13c30fa