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/sudoers.h

@@ -210,14 +210,6 @@ struct sudo_user {
 #endif
 #define ROOT_GID	0
 
-/*
- * We used to use the system definition of PASS_MAX or _PASSWD_LEN,
- * but that caused problems with various alternate authentication
- * methods.  So, we just define our own and assume that it is >= the
- * system max.
- */
-#define SUDO_PASS_MAX	256
-
 struct lbuf;
 struct passwd;
 struct stat;
@@ -287,9 +279,6 @@ void dump_auth_methods(void);
 /* getspwuid.c */
 char *sudo_getepw(const struct passwd *);
 
-/* zero_bytes.c */
-void zero_bytes(volatile void *, size_t);
-
 /* sudo_nss.c */
 void display_privs(struct sudo_nss_list *, struct passwd *);
 bool display_cmnd(struct sudo_nss_list *, struct passwd *);