all: make "ipv6.addr-gen-mode" configurable by global default

It can be useful to choose a different "ipv6.addr-gen-mode". And it can be
useful to override the default for a set of profiles.

For example, in cloud or in a data center, stable-privacy might not be
the best choice. Add a mechanism to override the default via global defaults
in NetworkManager.conf:

  # /etc/NetworkManager/conf.d/90-ipv6-addr-gen-mode-override.conf

"ipv6.addr-gen-mode" is a special property, because its default depends on
the component that configures the profile.

- when read from disk (keyfile and ifcfg-rh), a missing addr-gen-mode
  key means to default to "eui64".
- when configured via D-Bus, a missing addr-gen-mode property means to
  default to "stable-privacy".
- libnm's ip6-config::addr-gen-mode property defaults to
- when some tool creates a profile, they either can explicitly
  set the mode, or they get the default of the underlying mechanisms

  - nm-initrd-generator explicitly sets "eui64" for profiles it creates.
  - nmcli doesn' explicitly set it, but inherits the default form
    libnm's ip6-config::addr-gen-mode.
  - when NM creates a auto-default-connection for ethernet ("Wired connection 1"),
    it inherits the default from libnm's ip6-config::addr-gen-mode.

Global connection defaults only take effect when the per-profile
value is set to a special default/unset value. To account for the
different cases above, we add two such special values: "default" and
"default-or-eui64". That's something we didn't do before, but it seams
useful and easy to understand.

Also, this neatly expresses the current behaviors we already have. E.g.
if you don't specify the "addr-gen-mode" in a keyfile, "default-or-eui64"
is a pretty clear thing.

Note that usually we cannot change default values, in particular not for
libnm's properties. That is because we don't serialize the default
values to D-Bus/keyfile, so if we change the default, we change
behavior. Here we change from "stable-privacy" to "default" and
from "eui64" to "default-or-eui64". That means, the user only experiences
a change in behavior, if they have a ".conf" file that overrides the default.


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