gh-84570: Send-Wait Fixes for _xxinterpchannels (gh-111006)

There were a few things I did in gh-110565 that need to be fixed. I also forgot to add tests in that PR.

(Note that this PR exposes a refleak introduced by gh-110246. I'll take care of that separately.)

Include/internal/pycore_pythread.h

@@ -86,6 +86,21 @@ extern int _PyThread_at_fork_reinit(PyThread_type_lock *lock);
 #endif  /* HAVE_FORK */
 
 
+// unset: -1 seconds, in nanoseconds
+#define PyThread_UNSET_TIMEOUT ((_PyTime_t)(-1 * 1000 * 1000 * 1000))
+
+/* Helper to acquire an interruptible lock with a timeout.  If the lock acquire
+ * is interrupted, signal handlers are run, and if they raise an exception,
+ * PY_LOCK_INTR is returned.  Otherwise, PY_LOCK_ACQUIRED or PY_LOCK_FAILURE
+ * are returned, depending on whether the lock can be acquired within the
+ * timeout.
+ */
+// Exported for the _xxinterpchannels module.
+PyAPI_FUNC(PyLockStatus) PyThread_acquire_lock_timed_with_retries(
+    PyThread_type_lock,
+    PY_TIMEOUT_T microseconds);
+
+
 #ifdef __cplusplus
 }
 #endif