Code refactoring of 'alt-stack registration support'

Code refactoring of 'alt-stack registration support'

* darwin_stop_world.c (GC_stack_range_for, GC_push_all_stacks):
Reformat code dealing with alt-stack.
* pthread_stop_world.c (GC_push_all_stacks): Likewise.
* darwin_stop_world.c (GC_stack_range_for): Cast pointers to word when
compared.
* GC_push_all_stacks (GC_push_all_stacks): Likewise.
* include/gc.h (GC_register_altstack): Reformat comment; decorate with
GC_CALL; comment out argument names; change type of altstack_size and
stack_size to word.
* include/private/pthread_support.h (altstack, altstack_size, stack,
stack_size): Refine and reformat comment.
* include/private/pthread_support.h (altstack_size, stack_size): Change
type from int to word.
* pthread_support.c (main_stack_size, main_altstack_size): Likewise.
* pthread_support.c (main_pthread_self): Rename to main_pthread_id.
* pthread_support.c (GC_register_altstack): Decorate with GC_API and
GC_CALL.
* win32_threads.c (GC_register_altstack): Likewise.
* pthread_support.c (GC_register_altstack): Rename "thread" local
variable to "me"; new local variable "self"; use DCL_LOCK_STATE;
reformat code.
* pthread_support.c (GC_thr_init): New local variable "self"; compare
thread id using THREAD_EQUAL; reformat code.
* win32_threads.c (GC_register_altstack): Add GC_ATTR_UNUSED to all
arguments to avoid compiler warning (while the function is
unimplemented); add TODO.