Parse IPv6 aliases from FreeBSD rc configuration correctly

Summary:
After rebooting WindRiver, the IPv6 alias for the IRC bots was not configured.

Use ifconfig_<interface>_alias<number> instead of
<ifconfig_<interface>_ipv6_alias<number>, as /etc/network.subr only looks
for the former when parsing interface aliases:

for alias in `list_vars ifconfig_${_vif}_alias[0-9]\* |
    sort_lite -nk1.$((9+${#_vif}+7))`
do
    eval ifconfig_args=\"\$$alias\"
    # ... logic to check for "inet6" keyword in $ifconfig_args ...
done

Start IPv6 aliases at index 600 to avoid conflicts with IPv4 aliases.

As there is no plan to run a massive NAT gateway or a CDN edge node,
the 600 offset is perfectly safe for the netif templates.

References:

• https://docs.freebsd.org/en/books/handbook/network/
• /etc/network.subr from FreeBSD 15.1

Differential Revision: https://devcentral.nasqueron.org/D4213