Use "bool" in more spots in TUI

Use "bool" in more spots in TUI

This changes a few spots in the TUI to use "bool" rather than "int".
Tested on x86-64 Fedora 28.

gdb/ChangeLog
2019-12-30  Tom Tromey  <tom@tromey.com>

	* tui/tui-interp.c (tui_start_enabled): Now bool.
	(_initialize_tui_interp): Update.
	* tui/tui-hooks.c (tui_refreshing_registers): Now bool.
	(tui_register_changed)
	(tui_refresh_frame_and_register_information): Update.
	* tui/tui-win.c (tui_update_variables): Return bool.
	* tui/tui-win.h (tui_update_variables): Return bool.
	* tui/tui.c (tui_get_command_dimension): Return bool.
	* tui/tui.h (tui_get_command_dimension): Return bool.

Change-Id: I55b7f2d62d2ef88da3587dc914ada9f463ad8d2b