Parallel-marker code refactoring (replace GC_markers with GC_markers_m1)

Parallel-marker code refactoring (replace GC_markers with GC_markers_m1)

* include/private/gc_priv.h (GC_markers): Replace with GC_markers_m1
(which is GC_markers minus one); update the comment.
* mark.c (GC_markers): Likewise.
* mark.c (GC_help_marker): Replace GC_markers with GC_markers_m1.
* pthread_support.c (GC_is_mach_marker, start_mark_threads,
GC_segment_is_thread_stack, GC_fork_child_proc, GC_thr_init): Likewise.
* win32_threads.c (GC_get_next_stack, start_mark_threads,
GC_wait_marker, GC_notify_all_marker, GC_thr_init): Likewise.