Use getcurx in curses code

As suggested by Pedro, this changes a few spots to use getcurx, rather
than getyx.  This avoids some unused variable warnings.

2016-07-14  Tom Tromey  <tom@tromey.com>

	* tui/tui-winsource.c (tui_show_source_line): Use getcurx.
	* tui/tui-io.c (tui_puts): Use getcurx.
	(tui_redisplay_readline): Likewise.