Fix platform availability of some Unix constants (#10857)

stdlib/_curses.pyi

@@ -61,7 +61,8 @@ if sys.platform != "win32":
     A_DIM: int
     A_HORIZONTAL: int
     A_INVIS: int
-    A_ITALIC: int
+    if sys.platform != "darwin":
+        A_ITALIC: int
     A_LEFT: int
     A_LOW: int
     A_NORMAL: int

stdlib/os/__init__.pyi

@@ -70,9 +70,20 @@ if sys.platform != "win32":
         POSIX_FADV_WILLNEED: int
         POSIX_FADV_DONTNEED: int
 
-    SF_NODISKIO: int
-    SF_MNOWAIT: int
-    SF_SYNC: int
+    if sys.platform != "linux" and sys.platform != "darwin":
+        # In the os-module docs, these are marked as being available
+        # on "Unix, not Emscripten, not WASI."
+        # However, in the source code, a comment indicates they're "FreeBSD constants".
+        # sys.platform could have one of many values on a FreeBSD Python build,
+        # so the sys-module docs recommend doing `if sys.platform.startswith('freebsd')`
+        # to detect FreeBSD builds. Unfortunately that would be too dynamic
+        # for type checkers, however.
+        SF_NODISKIO: int
+        SF_MNOWAIT: int
+        SF_SYNC: int
+
+        if sys.version_info >= (3, 11):
+            SF_NOCACHE: int
 
     if sys.platform == "linux":
         XATTR_SIZE_MAX: int

stdlib/select.pyi

@@ -137,7 +137,6 @@ if sys.platform == "linux":
     EPOLLRDNORM: int
     EPOLLWRBAND: int
     EPOLLWRNORM: int
-    EPOLL_RDHUP: int
     EPOLL_CLOEXEC: int
 
 if sys.platform != "linux" and sys.platform != "darwin" and sys.platform != "win32":

stdlib/signal.pyi

@@ -21,8 +21,6 @@ class Signals(IntEnum):
         CTRL_C_EVENT: int
         CTRL_BREAK_EVENT: int
     else:
-        SIGEMT: int
-        SIGINFO: int
         SIGALRM: int
         SIGBUS: int
         SIGCHLD: int
@@ -47,6 +45,9 @@ class Signals(IntEnum):
         SIGWINCH: int
         SIGXCPU: int
         SIGXFSZ: int
+        if sys.platform != "linux":
+            SIGEMT: int
+            SIGINFO: int
         if sys.platform != "darwin":
             SIGCLD: int
             SIGPOLL: int
@@ -88,8 +89,9 @@ if sys.platform == "win32":
     CTRL_C_EVENT: Signals
     CTRL_BREAK_EVENT: Signals
 else:
-    SIGINFO: Signals
-    SIGEMT: Signals
+    if sys.platform != "linux":
+        SIGINFO: Signals
+        SIGEMT: Signals
     SIGALRM: Signals
     SIGBUS: Signals
     SIGCHLD: Signals