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).

src/sudo.c

@@ -282,6 +282,10 @@ main(int argc, char *argv[], char *envp[])
 	    iolog_open(settings, user_info, command_info, nargc, nargv,
 		user_env_out);
 
+	    /* Audit the accept event on behalf of the sudo front-end. */
+	    audit_accept("sudo", SUDO_FRONT_END, command_info,
+		nargv, user_env_out);
+
 	    /* Setup command details and run command/edit. */
 	    command_info_to_details(command_info, &command_details);
 	    command_details.tty = user_details.tty;