Use abstract SocketListener class

Makes the code more general and not directly tied to specifically
TCP sockets.
diff --git a/vncviewer/vncviewer.cxx b/vncviewer/vncviewer.cxx
index 413260d..f22648d 100644
--- a/vncviewer/vncviewer.cxx
+++ b/vncviewer/vncviewer.cxx
@@ -547,7 +547,7 @@
 #endif
 
   if (listenMode) {
-    std::list<TcpListener*> listeners;
+    std::list<SocketListener*> listeners;
     try {
       int port = 5500;
       if (isdigit(vncServerName[0]))
@@ -561,7 +561,7 @@
       while (sock == NULL) {
         fd_set rfds;
         FD_ZERO(&rfds);
-        for (std::list<TcpListener*>::iterator i = listeners.begin();
+        for (std::list<SocketListener*>::iterator i = listeners.begin();
              i != listeners.end();
              i++)
           FD_SET((*i)->getFd(), &rfds);
@@ -576,7 +576,7 @@
           }
         }
 
-        for (std::list<TcpListener*>::iterator i = listeners.begin ();
+        for (std::list<SocketListener*>::iterator i = listeners.begin ();
              i != listeners.end();
              i++)
           if (FD_ISSET((*i)->getFd(), &rfds)) {