roken: Add memset_s implementation

Add an implementation of memset_s to roken.

Some optimising compilers may remove the memset() instruction when it
is used immediately before a free, which defeats its purpose if the
intention is to zero memory before returning it to the heap or stack.

C11 added memset_s, provide a fallback in roken so that memset_s can
be used on all platforms.

lib/roken/version-script.map

@@ -80,6 +80,7 @@ HEIMDAL_ROKEN_1.0 {
 		rk_inet_ntop;
 		rk_inet_pton;
 		rk_localtime_r;
+		rk_memset_s;
 		rk_mkdir;
 		rk_mkstemp;
 		rk_pid_file_delete;