improve error message when we can't listen on port

ChangeLog

@@ -1,4 +1,5 @@
 Version NEXT
+	display port number if we can't listen on it
 
 Version 2.8.1b29 - 24 Aug 2019
         added scons command line argument without-fspscan=yes for building

server/main.c

@@ -243,7 +243,9 @@ int main (int argc, char ** argv)
   {
     opt=_x_udp(listen_on,&udp_port);
     if(opt == -1) {
-    perror("Error: socket open");
+    fprintf(stderr,"can't listen on port %d: ",udp_port);
+    fflush(stderr);
+    perror("");
     exit(2);
     }
     if(dbug) {