Introduce function for directly updating GDB's screen dimensions

Introduce function for directly updating GDB's screen dimensions

... to replace the roundabout pattern of

  execute_command ("set width %d");
  execute_command ("set height %d");

for doing the same thing.

gdb/ChangeLog:

	* utils.h (set_screen_width_and_height): Declare.
	* utils.c (set_screen_width_and_height): Define.
	* tui/tui-win.c (tui_update_gdb_sizes): Use it.