resolved: properly handle LLMNR/TCP connection errors

The LLMNR spec suggests to do do reverse address lookups by doing direct LLMNR/TCP connections to the indicated
address, instead of doing any LLMNR multicast queries. When we do this and the peer doesn't actually implement LLMNR
this will result in a TCP connection error, which we need to handle. In contrast to most LLMNR lookups this will give
us a quick response on whether we can find a suitable name. Report this as new transaction state, since this should
mostly be treated like an NXDOMAIN rcode, except that it's not one.

src/basic/fd-util.h

@@ -74,5 +74,6 @@ void cmsg_close_all(struct msghdr *mh);
 
 bool fdname_is_valid(const char *s);
 
+/* Hint: ENETUNREACH happens if we try to connect to "non-existing" special IP addresses, such as ::5 */
 #define ERRNO_IS_DISCONNECT(r) \
-        IN_SET(r, ENOTCONN, ECONNRESET, ECONNREFUSED, ECONNABORTED, EPIPE)
+        IN_SET(r, ENOTCONN, ECONNRESET, ECONNREFUSED, ECONNABORTED, EPIPE, ENETUNREACH)