Implemented `gc-stats' using `libgc' information.

Implemented `gc-stats' using `libgc' information.

* libguile/gc.c (protected_obj_count): Made `static'.
  (scm_heap_size): Mapped to `heap-size' rather than `cell-heap-size'.
  (scm_heap_free_size): New.
  (scm_heap_total_allocated): New.
  (scm_gc_stats): Rewritten.  Return the (little) information available
  from `libgc'.
  (scm_gc_disable): New.
  (scm_gc_enable): New.
  (scm_storage_prehistory): Don't call `GC_add_roots ()' with
  SCM_SYS_PROTECTS.  Use `GC_is_visible ()' to check whether
  SCM_SYS_PROTECTS is visible.

* libguile/gc.h (scm_gc_enable): New declaration.
  (scm_gc_disable): New declaration.
  (scm_gc_for_alloc): Removed.
  (scm_gc_for_newcell): Removed.

git-archimport-id: lcourtes@laas.fr--2005-libre/guile-core--boehm-gc--1.9--patch-46