Various error-handling fixes, mostly in Unix PuTTY but one (failure

to save a session) crosses over into the platform-independent side.

[originally from svn r3041]

unix/ptermm.c

@@ -3,6 +3,7 @@
  */
 
 #include <stdio.h>
+#include <stdlib.h>
 
 #include "putty.h"
 
@@ -16,6 +17,16 @@ int cfgbox(Config *cfg)
     return 1;			       /* no-op in pterm */
 }
 
+void fatal_message_box(void *window, char *msg)
+{
+    /* also a no-op in pterm */
+}
+
+void cleanup_exit(int code)
+{
+    exit(code);
+}
+
 int process_nonoption_arg(char *arg, Config *cfg)
 {
     return 0;                          /* pterm doesn't have any. */