The (deprecated but still available) properties gateway4 and gateway6
were not being taken into account when comparing routes. Because of
that, the diff report will show the default route as missing in netplan.

There is a complication when considering gateway4 and gateway6 though:
both networkd and NM will set a metric for them. As these properties
cannot set metrics, these routes would still be shown in the diff
report.

To avoid this noise, when gateway4 and/or gateway6 are used AND the
system contains one, and only one, default route per protocol family, we
adopt the same metric and route table from the existing routes. By doing
this, the comparison will see that both routes, from the system and from
netplan, are the same and not show them as a diff.

When available, use the flag "dynamic" from addr_info to check if the IP
addresses was dynamically assigned.

The flag will be displayed in the status output along other flags:

 Addresses: 10.86.126.222/24 (dynamic, dhcp)

There are cases where IPv6 addresses received via RA are not set as
'dynamic'. In these cases, use the information about routes to find
these IPs and add the tag 'ra' to them.

It's done by iterating through IPv6 addresses and checking if there is
any addresses in the same network as routes received via RA.

Add the Python binding and unit test.

NOTE

accept_ra is not perfectly mapped from the netdef structure to the
YAML setting and back to the renderer. It's a boolean value in the YAML
but it maps to three different values inside libnetplan: kernel, enabled
and disabled.

Not setting it in the YAML means it will not be set in
networkd. Although, networkd will disable it unconditionally in
the kernel and will itself handle RAs, meaning that when unset, it's
enabled by default (with a few exceptions, see systemd.network(5)).

Because of that, the low level C API will return the actual internal
value of the property and not only TRUE or FALSE. The Python code will
return True, False or None (when it's unset). This will help the
consumers to differentiate the case where it's disabled or unset.

When IPv6 link-local and RA (which depends on link-local) are enabled,
the system will receive IP addresses, nameservers and routes dynamically
via RA. These addresses are not obviously identified as dynamic in the
system. networkd probably does have this information but it's not fully
exposed apparently.

Here I try to improve the comparison of these addresses by checking if
they are 1) a link-local address and 2) part of the same network as the
route received via RA.

Without this, status --diff will show the RA IP addresses and link-local
DNS nameserver as a diff.

The logic to remove tags from the output wasn't working for the
interface header in diff mode.

When networkd is used, the ConfigSource property can identify the source
of the configuration so let's use it to be more precise and reduce the
guessing.

We consume the same information when Network Manager is the renderer but
in this case networkd will not know the sources so this property can't
be relied on.

For now, we collect the sources for IP addresses and DNS. Routes can be
included in the future. We also should create abstractions for each
configuration so we don't need to keep information such as the data
source in separate tables.

Network Manager seems to expose the source through its DBus API. This
also could be considered in the future.