cmdutils: move exit_program() declaration to cmdutils from avconv

Allows cmdutils to call each tool's own cleanup function.

avserver.c

@@ -320,6 +320,11 @@ static AVLFG random_state;
 
 static FILE *logfile = NULL;
 
+void exit_program(int ret)
+{
+    exit(ret);
+}
+
 /* FIXME: make avserver work with IPv6 */
 /* resolve host with also IP address parsing */
 static int resolve_host(struct in_addr *sin_addr, const char *hostname)