feat: add --without-ncurses option for daemon-only builds

Add a dummy curses stub header that provides all ncurses types, macros,
and no-op function stubs. When configured with --without-ncurses, the
build uses this stub instead of linking to the real ncurses library.

This allows rtorrent to be built for daemon-only usage (e.g. with
ruTorrent or Flood) without requiring ncurses to be installed.

Closes #1613

src/display/curses_stub.cc

@@ -0,0 +1,6 @@
+// Dummy ncurses stub: provides the stdscr symbol expected by the codebase.
+
+#include "curses_stub.h"
+
+// Define stdscr as a null pointer; all ncurses calls are no-ops in this stub.
+WINDOW* stdscr = nullptr;