Improve SIGINT handling in multi-threaded programs

The flag remembering whether an Interrupted exception was thrown is
now thread-local. Thus, all threads will (eventually) throw
Interrupted. Previously, one thread would throw Interrupted, and then
the other threads wouldn't see that they were supposed to quit.

src/libutil/util.hh

@@ -316,6 +316,8 @@ void restoreSIGPIPE();
 
 extern volatile sig_atomic_t _isInterrupted;
 
+extern thread_local bool interruptThrown;
+
 void _interrupted();
 
 void inline checkInterrupt()