Properly exit on SIGTERM

Previously we could get hung when killed, e.g. when using cu.
When using minicom or screen this problem did not seem to occur.

ttynvt.c

@@ -137,6 +137,8 @@ static void sig_handler(int sig)
 {
     DBG("Interruped, signal = %d\n", sig);
     _is_interrupted = 1;
+    if (sig != SIGUSR2)
+        exit(0);
 }
 
 static void set_signal_handler(void)
@@ -148,6 +150,7 @@ static void set_signal_handler(void)
     sigemptyset(&(sa.sa_mask));
     sa.sa_flags = 0;
     sigaction(SIGUSR2, &sa, NULL);
+    sigaction(SIGTERM, &sa, NULL);
 }
 
 static void ttynvt_interrupted(fuse_req_t req, void *data)