Add Posix.IO.dup2, as specified by Basis Library.

basis/POSIX_IO.sml

@@ -24,6 +24,7 @@ signature POSIX_IO =
 
     val close : file_desc -> unit
     val dup : file_desc -> file_desc
+    val dup2 : {old : file_desc, new : file_desc} -> unit
     val dupfd : {old : file_desc, base : file_desc} -> file_desc
     val pipe : unit -> {infd : file_desc, outfd : file_desc}
 

basis/Posix.sml

@@ -823,6 +823,10 @@ struct
 
     fun dup f = dupfd {old = f, base = 0}
 
+    fun dup2 {old, new} = let val a = prim ("@sml_dup2", (old : int, new : int)) : int
+                          in if a = ~1 then raiseSys "dup2" NONE "" else ()
+                          end
+
     fun lseek (fd, p, w) =
         let
           val k = case w of SEEK_SET => 0

src/Runtime/Posix.c

@@ -302,6 +302,12 @@ sml_dupfd(size_t f, size_t arg)
   return (size_t) fcntl((int) f, F_DUPFD, (long) arg);
 }
 
+size_t
+sml_dup2(size_t f, size_t t)
+{
+  return (size_t) dup2((int) f, (int) t);
+}
+
 uintptr_t
 sml_getStdNumbers(uintptr_t triple)
 {