MINOR: protocols: use is_inet_addr() when only INET addresses are desired

We used to have is_addr() in place to validate sometimes the existence
of an address, sometimes a valid IPv4 or IPv6 address. Replace them
carefully so that is_inet_addr() is used wherever we can only use an
IPv4/IPv6 address.
diff --git a/src/backend.c b/src/backend.c
index 9c3ae0e..0912fd4 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -753,7 +753,8 @@
 		if (!is_addr(&srv_conn->addr.to) && cli_conn) {
 			/* if the server has no address, we use the same address
 			 * the client asked, which is handy for remapping ports
-			 * locally on multiple addresses at once.
+			 * locally on multiple addresses at once. Nothing is done
+			 * for AF_UNIX addresses.
 			 */
 			conn_get_to_addr(cli_conn);