all: check return value from daemon() call

* */*main.c: (main) Current versions of Gcc warn if the return value for
  daemon() is not checked.  So add a simple test and exit on failure.

diff --git a/isisd/isis_main.c b/isisd/isis_main.c
index 2411518..c5e824c 100644
--- a/isisd/isis_main.c
+++ b/isisd/isis_main.c
@@ -333,8 +333,11 @@
     return(0);
   
   /* demonize */
-  if (daemon_mode)
-    daemon (0, 0);
+  if (daemon_mode && daemon (0, 0) < 0)
+    {
+      zlog_err("ISISd daemon failed: %s", strerror(errno));
+      exit (1);
+    }
 
   /* Process ID file creation. */
   pid_output (pid_file);