* async.c (scm_sys_tick_async_thunk): commented out. I'm not

	* async.c (scm_sys_tick_async_thunk): commented out.  I'm not
	sure how this was supposed to work.
	(scm_async_click): don't send SCM_TICK_SIGNAL.
	(scm_init_async): don't initialize %tick-thunk.

	* the following change doesn't affect the Scheme interface:
 	gc-thunk is called at the end of garbage collection.  however it's
 	no longer implemented by pretending it's a signal.

	* gc.c (scm_gc_end): don't call scm_take_signal.  instead mark the
	system async corresponding to scm_gc_thunk.
	* async.h: declare scm_gc_async.
	* async.c (scm_sys_gc_async_thunk): apply the thunk named by
	gc-thunk directly, instead of going through a signal handler.
	(scm_gc_async): new variable, points to the GC system-async.
	(scm_init_async): save the GC async as scm_gc_async instead
	of using system_signal_asyncs.
	(scm_gc_vcell): new variable, stores the gc-thunk vcell.