MINOR: unix: don't mention free ports on EAGAIN

When a connect() to a unix socket returns EAGAIN we talk about
"no free ports" in the error/debug message, which only makes
sense when using TCP.

Explain connect() failure and suggest troubleshooting server
backlog size.
(cherry picked from commit 9f256d4d85f2b9c79b0d4f28a27731260eefa1b2)
(cherry picked from commit ffcc8a9e05caec1def8db8f5218e2ecad264a2d4)
diff --git a/src/proto_uxst.c b/src/proto_uxst.c
index adc1b46..8e3393d 100644
--- a/src/proto_uxst.c
+++ b/src/proto_uxst.c
@@ -504,7 +504,7 @@
 		else if (errno == EAGAIN || errno == EADDRINUSE || errno == EADDRNOTAVAIL) {
 			char *msg;
 			if (errno == EAGAIN || errno == EADDRNOTAVAIL) {
-				msg = "no free ports";
+				msg = "can't connect to destination unix socket, check backlog size on the server";
 				conn->err_code = CO_ER_FREE_PORTS;
 			}
 			else {