Factor out duplicate signal handling code

No options are set for SIGTERM, so we can use the simpler
signal handling interface and remove unnecessary structure initilization.

sigemptyset() is moved outside of the ifdef to prevent any potential
unused variable warning.

Closes #1637

src/redis.c

@@ -3420,17 +3420,12 @@ static void sigShutdownHandler(int sig) {
 
 void setupSignalHandlers(void) {
     struct sigaction act;
-
-    /* When the SA_SIGINFO flag is set in sa_flags then sa_sigaction is used.
-     * Otherwise, sa_handler is used. */
     sigemptyset(&act.sa_mask);
-    act.sa_flags = 0;
-    act.sa_handler = sigShutdownHandler;
-    sigaction(SIGTERM, &act, NULL);
+
+    signal(SIGTERM, sigShutdownHandler);
     signal(SIGINT, sigShutdownHandler);
 
 #ifdef HAVE_BACKTRACE
-    sigemptyset(&act.sa_mask);
     act.sa_flags = SA_NODEFER | SA_RESETHAND | SA_SIGINFO;
     act.sa_sigaction = sigsegvHandler;
     sigaction(SIGSEGV, &act, NULL);