Fix multiprocessing.synchronize.SemLock.__setstate__ to properly initialize multiprocessing.synchronize.SemLock._is_fork_ctx. This fixes a regression when passing a SemLock accross nested processes.
Rename multiprocessing.synchronize.SemLock.is_fork_ctx to multiprocessing.synchronize.SemLock._is_fork_ctx to avoid exposing it as public API.