daemon, generator: Use power of 2 for initial size of Hashtbl.create

Before 2011 it was recommended to use a prime number for the initial
size.  In 2011 the OCaml hash table was reimplemented using a hash
function based on Murmur 3.  Hashtbl.create now adjusts the initial
size to the next power of 2 (minimum 16).  So replace obsolete
'Hashtbl.create 13' with 'Hashtbl.create 16'.

daemon/selinux.ml

@@ -35,7 +35,7 @@ let setfiles_has_option =
     let _, _, err = commandr "setfiles" [opt] in
     String.find err err_msg = -1
   in
-  let h = Hashtbl.create 13 in
+  let h = Hashtbl.create 16 in
   fun flag ->
     try Hashtbl.find h flag
     with