vtysh: drop unused variables & RETSIGTYPE

Drop unused return values in vtysh.  Also gets rid of the rather funny
prototyping of signal setup in vtysh - which as a side effect makes it
not need AC_TYPE_SIGNAL in configure.ac anymore.  It wasn't used
sensibly to begin with...

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/vtysh/vtysh_main.c b/vtysh/vtysh_main.c
index d243670..aa7d021 100644
--- a/vtysh/vtysh_main.c
+++ b/vtysh/vtysh_main.c
@@ -98,10 +98,9 @@
 
 /* Signale wrapper for vtysh. We don't use sigevent because
  * vtysh doesn't use threads. TODO */
-RETSIGTYPE *
+static void
 vtysh_signal_set (int signo, void (*func)(int))
 {
-  int ret;
   struct sigaction sig;
   struct sigaction osig;
 
@@ -112,12 +111,7 @@
   sig.sa_flags |= SA_RESTART;
 #endif /* SA_RESTART */
 
-  ret = sigaction (signo, &sig, &osig);
-
-  if (ret < 0) 
-    return (SIG_ERR);
-  else
-    return (osig.sa_handler);
+  sigaction (signo, &sig, &osig);
 }
 
 /* Initialization of signal handles. */