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/logging.c

@@ -609,7 +609,7 @@ send_mail(const char *fmt, ...)
     closefrom(STDERR_FILENO + 1);
 
     /* Ignore SIGPIPE in case mailer exits prematurely (or is missing). */
-    zero_bytes(&sa, sizeof(sa));
+    memset(&sa, 0, sizeof(sa));
     sigemptyset(&sa.sa_mask);
     sa.sa_flags = SA_INTERRUPT;
     sa.sa_handler = SIG_IGN;