* shm.cc: Include sync.h

	* shm.cc: Include sync.h
	(struct shm_shmid_list): Add ref_count member.
	(struct shm_attached_list): Remove hdl and size members.  Add a parent
	member pointing to referenced shm_shmid_list entry.
	(shm_guard): New muto.
	(SLIST_LOCK): Define.
	(SLIST_UNLOCK): Define.
	(fixup_shms_after_fork): Use hdl and size members of parent
	shm_shmid_list entry.
	(shmat): Access sequential bookkeeping lists in a thread safe way.
	Accommodate change in list element layout.  Align comments.
	(shmctl): Ditto.
	(shmdt): Ditto.
	(shmget): Ditto.