Add more constants and attributes to posix (#4552)

The constants were found by running dir(posix) on a Debian 10.5 (x86_64)
system.

Resolves #616.

Author: srittau

stdlib/3/posix.pyi

@@ -1,10 +1,6 @@
-# Stubs for posix
-
-# NOTE: These are incomplete!
-
 import sys
 from os import stat_result as stat_result
-from typing import List, NamedTuple, Optional, overload
+from typing import Any, Dict, List, NamedTuple, Optional, overload
 
 if sys.version_info >= (3, 6):
     from builtins import _PathLike  # See comment in builtins
@@ -33,6 +29,11 @@ class waitid_result(NamedTuple):
 class sched_param(NamedTuple):
     sched_priority: int
 
+CLD_CONTINUED: int
+CLD_DUMPED: int
+CLD_EXITED: int
+CLD_TRAPPED: int
+
 EX_CANTCREAT: int
 EX_CONFIG: int
 EX_DATAERR: int
@@ -56,6 +57,11 @@ R_OK: int
 W_OK: int
 X_OK: int
 
+F_LOCK: int
+F_TEST: int
+F_TLOCK: int
+F_ULOCK: int
+
 if sys.version_info >= (3, 6):
     GRND_NONBLOCK: int
     GRND_RANDOM: int
@@ -82,6 +88,39 @@ O_SYNC: int
 O_TRUNC: int
 O_WRONLY: int
 
+POSIX_FADV_DONTNEED: int
+POSIX_FADV_NOREUSE: int
+POSIX_FADV_NORMAL: int
+POSIX_FADV_RANDOM: int
+POSIX_FADV_SEQUENTIAL: int
+POSIX_FADV_WILLNEED: int
+
+PRIO_PGRP: int
+PRIO_PROCESS: int
+PRIO_USER: int
+
+P_ALL: int
+P_PGID: int
+P_PID: int
+
+RTLD_DEEPBIND: int
+RTLD_GLOBAL: int
+RTLD_LAZY: int
+RTLD_LOCAL: int
+RTLD_NODELETE: int
+RTLD_NOLOAD: int
+RTLD_NOW: int
+
+SCHED_BATCH: int
+SCHED_FIFO: int
+SCHED_IDLE: int
+SCHED_OTHER: int
+SCHED_RESET_ON_FORK: int
+SCHED_RR: int
+
+SEEK_DATA: int
+SEEK_HOLE: int
+
 ST_APPEND: int
 ST_MANDLOCK: int
 ST_NOATIME: int
@@ -107,6 +146,10 @@ WSTOPSIG: int
 WTERMSIG: int
 WUNTRACED: int
 
+XATTR_CREATE: int
+XATTR_REPLACE: int
+XATTR_SIZE_MAX: int
+
 if sys.version_info >= (3, 6):
     @overload
     def listdir(path: Optional[str] = ...) -> List[str]: ...
@@ -124,3 +167,8 @@ else:
     def listdir(path: bytes) -> List[bytes]: ...
     @overload
     def listdir(path: int) -> List[str]: ...
+
+if sys.platform == "win32":
+    environ: Dict[str, str]
+else:
+    environ: Dict[bytes, bytes]