Add compiler attributes to type check format strings
Fix radvd.c:342:2: warning: too many arguments for format [-Wformat-extra-args]

Most likely a copy paste error.
Fix format string warnings

recv.c: In function ‘recv_rs_ra’:
recv.c:80:5: warning: format ‘%d’ expects argument of type ‘int’, but
argument 3 has type ‘size_t’ [-Wformat=]
     flog(LOG_ERR, "received a bogus IPV6_HOPLIMIT from the kernel!
     len=%d, data=%d", cmsg->cmsg_len, *(int *)CMSG_DATA(cmsg));
          ^
          recv.c:90:35: warning: format ‘%d’ expects argument of type
          ‘int’, but argument 3 has type ‘size_t’ [-Wformat=]
                    cmsg->cmsg_len, ((struct in6_pktinfo
                    *)CMSG_DATA(cmsg))->ipi6_ifindex);