rc: Improve netwait DAD logic

Disable if IPv6 is not supported, and instead of 10 seconds, default to
one more than the value of net.inet6.ip6.dad_count.

Fixes: 5ead817c3b7a ("rc: Teach netwait to wait for DAD")
Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D52905