Add DHCPv4 option 108 "ipv6-only" to the options table.

src/dhcp-common.c

@@ -685,6 +685,7 @@ static const struct opttab_t {
   { "client-machine-id", 97, 0 },
   { "posix-timezone", 100, OT_NAME }, /* RFC 4833, Sec. 2 */
   { "tzdb-timezone", 101, OT_NAME }, /* RFC 4833, Sec. 2 */
+  { "ipv6-only", 108, 4 | OT_DEC },  /* RFC 8925 */ 
   { "subnet-select", 118, OT_INTERNAL },
   { "domain-search", 119, OT_RFC1035_NAME },
   { "sip-server", 120, 0 },