internal/corerad: *BSD autoconf and forwarding configuration

[mdlayher]

After fixing a small compilation failure, it looks like CoreRAD works just fine on FreeBSD.

$ sudo tcpdump icmp6 -vvv
tcpdump: listening on vtnet0, link-type EN10MB (Ethernet), capture size 262144 bytes
15:21:50.906990 IP6 (hlim 255, next-header ICMPv6 (58) payload length: 160) fe80::a026:8aff:fe33:4e1 > ff02::1: [icmp6 sum ok] ICMP6, router advertisement, length 160
        hop limit 64, Flags [none], pref medium, router lifetime 1800s, reachable time 0s, retrans time 0s
          prefix info option (3), length 32 (4): 2604:a880:cad:d0::/64, Flags [onlink, auto], valid time 86400s, pref. time 14400s
            0x0000:  40c0 0001 5180 0000 3840 0000 0000 2604
            0x0010:  a880 0cad 00d0 0000 0000 0000 0000
          prefix info option (3), length 32 (4): 2001:db8::/64, Flags [onlink, auto], valid time 86400s, pref. time 14400s
            0x0000:  40c0 0001 5180 0000 3840 0000 0000 2001
            0x0010:  0db8 0000 0000 0000 0000 0000 0000
          rdnss option (25), length 40 (5):  lifetime 1200s, addr: 2001:db8::1 addr: 2001:db8::2
            0x0000:  0000 0000 04b0 2001 0db8 0000 0000 0000
            0x0010:  0000 0000 0001 2001 0db8 0000 0000 0000
            0x0020:  0000 0000 0002
          dnssl option (31), length 32 (4):  lifetime 1200s, domain(s): foo.example.com.
            0x0000:  0000 0000 04b0 0366 6f6f 0765 7861 6d70
            0x0010:  6c65 0363 6f6d 0000 0000 0000 0000
          source link-address option (1), length 8 (1): a2:26:8a:33:04:e1
            0x0000:  a226 8a33 04e1

We should add support for tuning *BSD kernel configuration as well.