Implement memset_s() and use it instead of zero_bytes().

A new constant, SUDO_CONV_REPL_MAX, is defined by the plugin
API as the max conversation reply length.  This constant can be
used as a max value for memset_s() when clearing passwords
filled in by the conversation function.

plugins/sudoers/auth/bsdauth.c

@@ -156,7 +156,7 @@ bsdauth_verify(struct passwd *pw, char *prompt, sudo_auth *auth)
 
     if (pass) {
 	authok = auth_userresponse(as, pass, 1);
-	zero_bytes(pass, strlen(pass));
+	memset_s(pass, SUDO_CONV_REPL_MAX, 0, strlen(pass));
     }
 
     /* restore old signal handler */