Handle edge cases in interface and routing config

[czeumer]

The pull requests fixes two issues with very special configurations:

• if more than one default route with different metrics is specified, the one with the highest metric is chosen
• if an interface has more than one address and the first routable address is not the first address entry a non routable address is returned.

Here an example of such a fringe configuration:

interface Config (excerpt):

10: vlan_1@nic: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 01:23:45:cb:2a:b6 brd ff:ff:ff:ff:ff:ff
    inet 169.254.95.54/16 metric 2048 brd 169.254.255.255 scope link vlan_1
       valid_lft forever preferred_lft forever
    inet 10.1.42.7/24 brd 10.1.42.255 scope global vlan_1
       valid_lft forever preferred_lft forever
11: vlan_2@nic: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 01:23:45:cb:2a:b6 brd ff:ff:ff:ff:ff:ff
    inet 169.254.26.132/16 metric 2048 brd 169.254.255.255 scope link vlan_2
       valid_lft forever preferred_lft forever
    inet 10.32.1.133/25 brd 10.32.1.255 scope global vlan_2
       valid_lft forever preferred_lft forever

routes (excerpt):

default via 10.1.42.1 dev vlan_transfer proto static metric 40 
default via 192.168.155.1 dev eno5 proto dhcp src 192.168.155.12 metric 100 

With this configuration, the source address for the interfaces 10 and 11 will return the link local address of the interface.
The default route will return 192.168.155.1 which is only the second in the list.

Both issues are addressed in this pull request.

Cheers,
Carsten

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