SystemExec try SIGINT before SIGKILL

Even though SIGINT is intended to be sent from terminals only,
it is more successful at terminating various child processes than
closing stdin and sending SIGTERM.

libs/pbd/system_exec.cc

@@ -682,6 +682,13 @@ SystemExec::terminate ()
 		wait(WNOHANG);
 	}
 
+	if (pid) {
+		::kill(pid, SIGINT);
+		::usleep(250000);
+		sched_yield();
+		wait(WNOHANG);
+	}
+
 	/* if pid is non-zero, the child task is STILL executing after being
 	 * sent SIGTERM. Act tough ... send SIGKILL
 	 */