* gc.h, gc.c (scm_i_gc_admin_mutex): New, to protect

* gc.h, gc.c (scm_i_gc_admin_mutex): New, to protect
scm_gc_mallocated, for now.
(scm_init_storage): Initialize it.
* gc-malloc.c (descrease_mtrigger, increase_mtrigger): Use it.

* gc-mark.c (scm_gc_mark_dependencies): Call scm_i_string_mark,
scm_i_stringbuf_mark and scm_i_symbol_mark, as appropriate.
* gc-card.c (scm_i_sweep_card):  Call scm_i_string_free,
scm_i_stringbuf_free and scm_i_symbol_free, as appropriate.