* Problem

  When multiple DHCP clients attempt to install an IPv4 default route into the main table,
netlink.RouteAdd can fail with EEXIST. This leads to noisy errors and nondeterministic
behavior regarding which interface "owns" the default route.

* Summary of changes

1. internal/server/ip:
  - Made Route.List device-optional: when DevName is empty or unresolved, the OIF filter
    is cleared and routes across all devices are returned.
  - Introduced a single builder netlinkRoute(mode) to construct netlink.Route for both
    apply (add/replace/delete) and query (list) paths.
2. incus/dhcp (DHCPv4 path in main_forknet.go):
  - Introduce a policy to keep a single default route in the main table, owned by the
    lexicographically smallest interface name among candidates.
  - Inspect current main table routes using ip.Route.List to find the current default
    route owner.
  - If the new interface name sorts before the current owner (or there is no owner),
    install/replace the default route via ip.Route.Replace. Otherwise, skip installing
    the default route for this interface.
  - Mark installed routes with Proto="dhcp".
  - Use Replace rather than Add to make route installation idempotent across renewals
    and avoid EEXIST.

* Behavioral impact:

  - Route.List now returns routes across all devices when DevName is empty (previously, it
    always filtered by device).
  - Add, Replace, Delete, and Flush continue to require a valid device and behave as before.

Signed-off-by: Andrey Mozharovsky <andrii.mozharovskyi@virtuozzo.com>