rc.d/netif: Don't DAD if lo0 is the only IPv6 IF

The code in rc.d/netif waiting for IPv6 Duplicate Address Detection if
any network interfaces support IPv6. Unfortunately, since lo0 *always*
has IPv6 enabled, this means unconditionally sleeping, even on systems
which have no external IPv6 interfaces.

Since we presume that there is little risk of a duplicate address being
assigned on lo0, amend the test to wait only if there is an interface
*other than lo0* which supports IPv6.

Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D40141