sys/netinet6: switch net.inet6.ip6.use_stableaddr to on by default

This change switches to using RFC 7217 algorithm as the default to
generate SLAAC addresses for IPv6 interfaces configured with
accept_rtadv.

Reviewed by: pouria, glebius, zlei
Approved by: zlei
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D55138