server: correctly adjudicate collision bind() of specific port

On Linux (at least) SO_REUSEADDR, which allows a new listener to
bind while an existing sock is in FIN-WAIT.  Apparently this allows
any number of sockets to bind(), but only when listen() to succeed.

Further, on Linux there is a known documented race condition which
can result in all listen() failing.  It isn't clear how to handle
this case without a potentially infinite loop, so ignore it.
If this happens, then eg. no PVA server will get port 5075.

So when probing for another listener, it is necessary to enter the
listening state.  When this fails, the socket is no longer usable
for another bind(), so it is necessary to allocate another for the
next attempt.

src/serverconn.cpp

@@ -408,18 +408,40 @@ ServIface::ServIface(const SockAddr &addr, server::Server::Pvt *server, bool fal
     server->acceptor_loop.assertInLoop();
     auto orig_port = bind_addr.port();
 
-    sock = evsocket(bind_addr.family(), SOCK_STREAM, 0);
-
-    if(evutil_make_listen_socket_reuseable(sock.sock))
-        log_warn_printf(connsetup, "Unable to make socket reusable%s", "\n");
-
     // try to bind to requested port, then fallback to a random port
     while(true) {
+
+        sock = evsocket(bind_addr.family(), SOCK_STREAM, 0);
+
+        if(evutil_make_listen_socket_reuseable(sock.sock))
+            log_warn_printf(connsetup, "Unable to make socket reusable%s", "\n");
+
         try {
             sock.bind(bind_addr);
+            // symantics of SO_REUSEADDR on *nix allow multiple bind() to success.
+            // however, only one listen() will succeed
+
+            const int backlog = 4;
+            auto list(evconnlistener_new(server->acceptor_loop.base, onConnS, this, LEV_OPT_DISABLED|LEV_OPT_CLOSE_ON_EXEC, backlog, sock.sock));
+            if(!list) {
+                int err = evutil_socket_geterror(sock);
+                throw std::system_error(err, std::system_category());
+            }
+            listener = evlisten(__FILE__, __LINE__, list);
+
+            // added in libevent 2.1.1
+#ifndef LEV_OPT_DISABLED
+#  define LEV_OPT_DISABLED 0
+#endif
+
+            if(!LEV_OPT_DISABLED)
+                evconnlistener_disable(listener.get());
+
         } catch(std::system_error& e) {
-            if(fallback && e.code().value()==SOCK_EADDRINUSE) {
-                log_debug_printf(connsetup, "Address %s in use\n", bind_addr.tostring().c_str());
+            if(fallback && (e.code().value()==SOCK_EADDRINUSE || e.code().value()==SOCK_EACCES)) {
+                log_debug_printf(connsetup, "Address %s in use or not permitted: %s\n",
+                                 bind_addr.tostring().c_str(),
+                                 e.what());
                 bind_addr.setPort(0);
                 fallback = false;
                 continue;
@@ -438,18 +460,6 @@ ServIface::ServIface(const SockAddr &addr, server::Server::Pvt *server, bool fal
     if(orig_port && bind_addr.port() != orig_port) {
         log_warn_printf(connsetup, "Server unable to bind port %u, falling back to %s\n", orig_port, name.c_str());
     }
-
-    // added in libevent 2.1.1
-#ifndef LEV_OPT_DISABLED
-#  define LEV_OPT_DISABLED 0
-#endif
-
-    const int backlog = 4;
-    listener = evlisten(__FILE__, __LINE__,
-                        evconnlistener_new(server->acceptor_loop.base, onConnS, this, LEV_OPT_DISABLED|LEV_OPT_CLOSE_ON_EXEC, backlog, sock.sock));
-
-    if(!LEV_OPT_DISABLED)
-        evconnlistener_disable(listener.get());
 }
 
 void ServIface::onConnS(struct evconnlistener *listener, evutil_socket_t sock, struct sockaddr *peer, int socklen, void *raw)