Add refcnt_shared() and refcnt_read()

Add refcnt_shared() and refcnt_read()

refcnt_shared() checks whether the object has multiple references.
When refcnt_shared() returns zero, the caller is the only reference
holder.

refcnt_read() returns a snapshot of the counter value.

refcnt_shared() suggested by dlg@.

OK dlg@ mvs@