ipv6: report disabled for per-interface kernel disable

The kernel can disable IPv6 on a single interface via
net.ipv6.conf.<iface>.disable_ipv6=1 while the global stack stays
present. The NM backend still maps method=auto to enabled=true and wins
the merge, so show reported ipv6.enabled: true for such interfaces.

Read the per-interface sysctl on the show path and force matching
interfaces off.

Signed-off-by: Josephine Pfeiffer <josie@redhat.com>