Replace strerror_r with mxs_strerror

The mxs_strerror function requires no local buffer, thus making it simpler
and cleaner to use.

server/modules/protocol/maxscaled/maxscaled.c

@@ -150,10 +150,8 @@ static bool authenticate_socket(MAXSCALED *protocol, DCB *dcb)
     }
     else
     {
-        char errbuf[MXS_STRERROR_BUFLEN];
-
         MXS_ERROR("Could not get socket family of client connection: %s",
-                  strerror_r(errno, errbuf, sizeof(errbuf)));
+                  mxs_strerror(errno));
     }
 
     return authenticated;