treewide: Introduce 'local mode' for disconnected setups

There are setups where no host interface is available or configured
at all, intentionally or not, temporarily or not, but users expect
(Podman) containers to run in any case as they did with slirp4netns,
and we're now getting reports that we broke such setups at a rather
alarming rate.

To this end, if we don't find any usable host interface, instead of
exiting:

- for IPv4, use 169.254.2.1 as guest/container address and 169.254.2.2
  as default gateway

- for IPv6, don't assign any address (forcibly disable DHCPv6), and
  use the *first* link-local address we observe to represent the
  guest/container. Advertise fe80::1 as default gateway

- use 'tap0' as default interface name for pasta

Change ifi4 and ifi6 in struct ctx to int and accept a special -1
value meaning that no host interface was selected, but the IP family
is enabled. The fact that the kernel uses unsigned int values for
those is not an issue as 1. one can't create so many interfaces
anyway and 2. we otherwise handle those values transparently.

Fix a botched conditional in conf_print() to actually skip printing
DHCPv6 information if DHCPv6 is disabled (and skip printing NDP
information if NDP is disabled).

Link: https://github.com/containers/podman/issues/24614
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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