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
trim21