Add more multiprocessing function stubs (#1435)

Fixes #1422

stdlib/3/multiprocessing/__init__.pyi

@@ -1,11 +1,13 @@
 # Stubs for multiprocessing
 
-from typing import Any, Callable, Iterable, Mapping, Optional, Dict, List
+from typing import Any, Callable, Iterable, Mapping, Optional, Dict, List, Union
 
+from logging import Logger
 from multiprocessing.context import BaseContext
 from multiprocessing.managers import SyncManager
 from multiprocessing.pool import AsyncResult
 from multiprocessing.process import current_process as current_process
+import sys
 
 class Lock():
     def acquire(self, block: bool = ..., timeout: int = ...) -> None: ...
@@ -101,6 +103,18 @@ class Value():
     def __init__(self, typecode_or_type: str, *args: Any, lock: bool = ...) -> None: ...
 
 # ----- multiprocessing function stubs -----
+def active_children() -> List[Process]: ...
+def allow_connection_pickling() -> None: ...
 def cpu_count() -> int: ...
 def freeze_support() -> None: ...
+def get_logger() -> Logger: ...
+def log_to_stderr(level: Optional[Union[str, int]] = ...) -> Logger: ...
 def Manager() -> SyncManager: ...
+def set_forkserver_preload(module_names: List[str]) -> None: ...
+if sys.platform == 'win32' or sys.version_info >= (3, 4):
+    def set_executable(executable: str) -> None: ...
+if sys.version_info >= (3, 4):
+    def get_all_start_methods() -> List[str]: ...
+    def get_context(method: Optional[str] = ...) -> BaseContext: ...
+    def get_start_method(allow_none: Optional[bool]) -> Optional[str]: ...
+    def set_start_method(method: str, force: Optional[bool] = ...) -> None: ...