Introduce net.inet6.icmp6.reply_from_interface and net.inet6.icmp6.reply_src sysctls, which is equivalent to IPv4's net.inet.icmp.reply_from_interface and "net.inet.icmp.reply_src respectively.
Submitted by: Neel Chauhan <neel AT neelc DOT org>
PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=246748