init: Signal m_tip_block_cv on Ctrl-C

Signal m_tip_block_cv when Ctrl-C is pressed or SIGTERM is received, the same
way it is currently signalled when the `stop` RPC is called. This lets RPC
calls like `waitforblockheight` and IPC calls like `waitTipChanged` be
interrupted, instead of waiting for their original timeouts and delaying
shutdown.

Historical notes:

- The behavior where `stop` RPC signals `m_tip_block_cv`, but CTRL-C does not,
  has been around since the condition variable was introduced in #30409
  (7eccdaf160).
- The signaling was later moved without changing behavior in #30967
  (5ca28ef28b). This commit moves it again to
  the Interrupt() function, which is probably the place it should have been
  added initially, so it works for Ctrl-C shutdowns as well as `stop`
  shutdowns.
- A Qt shutdown bug calling wait methods was fixed previously in #18452
  (da73f1513a), and this change updates that
  fix to avoid the hang happening again in Qt.

src/init.cpp

@@ -215,8 +215,6 @@ void InitContext(NodeContext& node)
     node.shutdown_request = [&node] {
         assert(node.shutdown_signal);
         if (!(*node.shutdown_signal)()) return false;
-        // Wake any threads that may be waiting for the tip to change.
-        if (node.notifications) WITH_LOCK(node.notifications->m_tip_block_mutex, node.notifications->m_tip_block_cv.notify_all());
         return true;
     };
 }
@@ -267,6 +265,8 @@ void Interrupt(NodeContext& node)
 #if HAVE_SYSTEM
     ShutdownNotify(*node.args);
 #endif
+    // Wake any threads that may be waiting for the tip to change.
+    if (node.notifications) WITH_LOCK(node.notifications->m_tip_block_mutex, node.notifications->m_tip_block_cv.notify_all());
     InterruptHTTPServer();
     InterruptHTTPRPC();
     InterruptRPC();