Use SUDO_CONV_DEBUG_MSG in the plugin instead of writing

directly to the debug file.

src/conversation.c

@@ -47,9 +47,12 @@
 
 #include "sudo.h"
 #include "sudo_plugin.h"
+#include "sudo_plugin_int.h"
 
 extern int tgetpass_flags; /* XXX */
 
+sudo_conv_t sudo_conv;	/* NULL in sudo front-end */
+
 /*
  * Sudo conversation function.
  */