rc: Teach netwait to wait for DAD

In some configurations, especially in jails, it is possible for the
system to boot so fast that we end up launching daemons while duplicate
address detection is still ongoing. If that happens, said daemons may
fail to bind to IPv6 addresses, as they are still tentative. Teach the
netwait service to wait (up to 10 seconds, by default) for the tentative
flag to vanish.

MFC after: 1 week
Reviewed by: olce
Differential Revision: https://reviews.freebsd.org/D51889