Use the term `scroll bar', instead of `scrollbar'.

Use the term `scroll bar', instead of `scrollbar'.
	* alloc.c, frame.c, frame.h, indent.c, keyboard.c, keyboard.h,
	lisp.h, term.c, termhooks.h, window.c, window.h, xdisp.c, xfns.c,
	xterm.c, xterm.h: Terminology changed.

	* xterm.c (x_window_to_scrollbar, x_scrollbar_expose,
	x_scrollbar_handle_click, x_scrollbar_note_movement): Remember
	that these can be called during GC; we have to ignore mark bits.
	* lisp.h (GC_NILP, GC_EQ): New macros to help with that.