Modify the behaviour of --synth-domain for IPv6.

When deriving a domain name from an IPv6 address, an address
such as 1234:: would become 1234--.example.com, which is
not legal in IDNA2008. Stop using the :: compression method,
so 1234:: becomes
1234-0000-0000-0000-0000-0000-0000-0000.example.com

src/option.c

@@ -2636,8 +2636,11 @@ static int one_opt(int option, char *arg, char *errstr, char *gen_err, int comma
 			  {
 			    if (option != 's')
 			      {
+				/* IPv6 address is longest and represented as
+				   xxxx-xxxx-xxxx-xxxx-xxxx-xxxx-xxxx-xxxx
+				   which is 39 chars */
 				if (!(new->prefix = canonicalise_opt(arg)) ||
-				    strlen(new->prefix) > MAXLABEL - INET_ADDRSTRLEN)
+				    strlen(new->prefix) > (MAXLABEL - 39))
 				  ret_err_free(_("bad prefix"), new);
 			      }
 			    else if (strcmp(arg, "local") != 0)