ask which source to open when --server is absent

The chooser runs before ScopeModel exists, so declining costs no buffer and no
socket. UDPSCOPE_PROBE_SOURCES=1 prints what the discovery found and exits,
before gtk_init(), so the list is checkable with no display.

src/config.cpp

@@ -67,6 +67,7 @@ Config parse_args(int argc, char** argv) {
       std::exit(0);
     } else if (a == "--server" || a.rfind("--server=", 0) == 0) {
       c.server = value_of(argc, argv, i, "--server");
+      c.server_specified = true;
     } else if (a == "--port" || a.rfind("--port=", 0) == 0) {
       const std::string v = value_of(argc, argv, i, "--port");
       const long p = std::strtol(v.c_str(), nullptr, 10);