rename scm_async_click() to scm_async_tick(); privatize SCM_ASYNC_TICK

rename scm_async_click() to scm_async_tick(); privatize SCM_ASYNC_TICK

* libguile/_scm.h (SCM_ASYNC_TICK, SCM_ASYNC_TICK_WITH_CODE): Make these
  definitions private.  Call the tick() function instead of click().

* libguile/__scm.h:
* libguile/async.h (scm_async_tick): Move declaration here.  Remove
  scm_async_click declaration.  Tick is the new (and old!) click, you
  see.
  (SCM_CRITICAL_SECTION_END): Call tick() instead of click().

* libguile/async.c (scm_async_tick): Remove old definition, and rename
  scm_async_click to scm_async_tick.
  (decrease_block): Adapt to click() -> tick() name change.

* libguile/deprecated.h (scm_async_click, SCM_ASYNC_TICK): Define some
  GONE macros.

* libguile/threads.c (fat_mutex_unlock): Tick() instead of click().