Add support for "accept" audit events sent by the sudo front-end.

With this change, the sudo front-end will send an "accept" audit
event to the audit plugins after all the I/O logging plugins have
been initialized.  This can be used by an audit plugin that does
not care about the result of the individual policy and approval
plugins and only wants to receive a single "accept" event if all
policy and approval plugins have succeeded.  The plugin_type argument
for events sent by the front-end is SUDO_FRONT_END (0).

include/sudo_plugin.h

@@ -35,6 +35,9 @@
     *(vp) = (*(vp) & 0xffff0000U) | (n); \
 } while(0)
 
+/* "plugin type" for the sudo front end, as passed to an audit plugin */
+#define SUDO_FRONT_END		0
+
 /* Conversation function types and defines */
 struct sudo_conv_message {
 #define SUDO_CONV_PROMPT_ECHO_OFF   0x0001  /* do not echo user input */