fcntl: various improvements (#3680)

* fcntl: mark positional-only args

* fcntl: use overload for fcntl.fcntl

The comment about depending on the type of arg seems incorrect
bf501353a0

I checked the docs and examples, CPython implementation and CPython tests, but
I might be missing something

* fcntl: use overload for fcntl.ioctl

Based off of docs and examples

* fcntl: type buffers better

Follows the approach in #2610

stdlib/3/fcntl.pyi

@@ -1,7 +1,13 @@
 # Stubs for fcntl
+from array import array
 from io import IOBase
-from typing import Any, IO, Union
+from typing import IO, Any, Union, overload
 from _types import FileDescriptorLike
+import sys
+if sys.version_info >= (3, 8):
+    from typing import Literal
+else:
+    from typing_extensions import Literal
 
 FASYNC: int
 FD_CLOEXEC: int
@@ -76,20 +82,39 @@ LOCK_SH: int
 LOCK_UN: int
 LOCK_WRITE: int
 
-# TODO All these return either int or bytes depending on the value of
-# cmd (not on the type of arg).
-def fcntl(fd: FileDescriptorLike,
-          cmd: int,
-          arg: Union[int, bytes] = ...) -> Any: ...
-# TODO This function accepts any object supporting a buffer interface,
-# as arg, is there a better way to express this than bytes?
-def ioctl(fd: FileDescriptorLike,
-          request: int,
-          arg: Union[int, bytes] = ...,
-          mutate_flag: bool = ...) -> Any: ...
-def flock(fd: FileDescriptorLike, operation: int) -> None: ...
-def lockf(fd: FileDescriptorLike,
-          cmd: int,
-          len: int = ...,
-          start: int = ...,
-          whence: int = ...) -> Any: ...
+@overload
+def fcntl(__fd: FileDescriptorLike,
+          __cmd: int,
+          __arg: int = ...) -> int: ...
+@overload
+def fcntl(__fd: FileDescriptorLike,
+          __cmd: int,
+          __arg: bytes) -> bytes: ...
+_ReadOnlyBuffer = bytes
+_WritableBuffer = Union[bytearray, memoryview, array]
+@overload
+def ioctl(__fd: FileDescriptorLike,
+          __request: int,
+          __arg: int = ...,
+          __mutate_flag: bool = ...) -> int: ...
+@overload
+def ioctl(__fd: FileDescriptorLike,
+          __request: int,
+          __arg: _WritableBuffer,
+          __mutate_flag: Literal[True] = ...) -> int: ...
+@overload
+def ioctl(__fd: FileDescriptorLike,
+          __request: int,
+          __arg: _WritableBuffer,
+          __mutate_flag: Literal[False]) -> bytes: ...
+@overload
+def ioctl(__fd: FileDescriptorLike,
+          __request: int,
+          __arg: _ReadOnlyBuffer,
+          __mutate_flag: bool) -> bytes: ...
+def flock(__fd: FileDescriptorLike, __operation: int) -> None: ...
+def lockf(__fd: FileDescriptorLike,
+          __cmd: int,
+          __len: int = ...,
+          __start: int = ...,
+          __whence: int = ...) -> Any: ...