* gc.c, gc.h (scm_i_sweep_mutex): New mutex.

* gc.c, gc.h (scm_i_sweep_mutex): New mutex.

* gc.c (scm_gc_for_newcell), gc-malloc.c (scm_realloc,
scm_gc_register_collectable_memory): Substitute locking of
scm_i_sweep_mutex for calls to scm_i_thread_put_to_sleep.
(scm_igc): Lock sweep mutex here instead of in callers; Calls to
scm_i_thread_put_to_sleep/scm_i_thread_wake_up used to demarkate
the single-thread section (which now only contains the mark
phase).
(scm_gc): Don't lock sweeo mutex here since scm_igc locks it;
Removed SCM_DEFER/ALLOW_INTS.  Simply call scm_igc directly.

* threads.c (gc_section_mutex): Removed.