Update help strings in TUI

This updates some help strings in the TUI to more closely follow GNU
standards.  In this case I chose to reuse some existing "usage" macros
in the help text.  Also, I found that XDBWIN_HEIGHT_USAGE is unused,
so I removed it.

ChangeLog
2018-04-29  Tom Tromey  <tom@tromey.com>

	* tui/tui-layout.c (_initialize_tui_layout): Update help text.
	* tui/tui-win.c (WIN_HEIGHT_USAGE, FOCUS_USAGE): Update
	(XDBWIN_HEIGHT_USAGE): Remove.
	(_initialize_tui_win): Use macros.  Update help text.