Revert "main-loop.c: Handle SIGINT, SIGHUP and SIGTERM synchronously"

This reverts commit 15124e1420. It breaks
debuggability of qemu and is no longer needed as the problem has
now been addressed in a different way.

Instead we provide a comment about why these signals must be
handled asynchronously.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>
[PMM: added comment]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

main-loop.c

@@ -84,9 +84,11 @@ static int qemu_signal_init(void)
     sigaddset(&set, SIGIO);
     sigaddset(&set, SIGALRM);
     sigaddset(&set, SIGBUS);
-    sigaddset(&set, SIGINT);
-    sigaddset(&set, SIGHUP);
-    sigaddset(&set, SIGTERM);
+    /* SIGINT cannot be handled via signalfd, so that ^C can be used
+     * to interrupt QEMU when it is being run under gdb.  SIGHUP and
+     * SIGTERM are also handled asynchronously, even though it is not
+     * strictly necessary, because they use the same handler as SIGINT.
+     */
     pthread_sigmask(SIG_BLOCK, &set, NULL);
 
     sigdelset(&set, SIG_IPI);