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.hpp

@@ -16,6 +16,11 @@ struct Config {
   std::string multicast;              ///< Empty selects unicast.
   std::string interface_addr;         ///< Local IPv4 to join the group on; empty = default route.
 
+  /// Whether `--server` was given. Absent, the application asks which source to open rather
+  /// than silently taking the default above -- so this is "did the user choose", not "is
+  /// `server` valid". The default stays a usable value for every other path.
+  bool server_specified = false;
+
   /// Size of *each* of the two sample buffers, in megabytes. The application
   /// always allocates two identical buffers (spec item 1).
   double buffer_mb = 200.0;