This change improves two parts for IPv6 configuration for FreeBSD 13 and later:
(1) it runs some more IPv6 configuration before IPv4 (rtsol before dhcp)
(2) it adds an RTOL pseudo-by running some IPv6 config keyword like DHCP to ifconfig_<ifN>_ipv6 in order to ruuration before IPv4 (rtsol now run
unconditionally on rtsol even if rtsold is not enabledd if we accept_rtadv and before dhcp).
MFC after: never