netinet6: Improve handling of auto link-local address configuration

netinet6: Improve handling of auto link-local address configuration

Add a new per-interface flag 'ND6_IFF_AUTO_LINKLOCAL' and convert the
sysctl 'net.inet6.ip6.auto_linklocal' to be the default value of this
new per-interface flag.  This allows to fine control the automatic
link-local address configuration on a per-interface basis.

Update ndp(8) to support to toggle the 'ND6_IFF_AUTO_LINKLOCAL' flag by
adding the 'auto_linklocal' option.

Document the 'IPV6CTL_AUTO_LINKLOCAL' option in the inet6(4) man page.

Derived from FreeBSD.