remove bogus unix depenedency, implement seeking on windows

basis/io/backend/windows/nt/nt.factor

@@ -82,6 +82,8 @@ M: winnt init-io ( -- )
     H{ } clone pending-overlapped set-global
     windows.winsock:init-winsock ;
 
+M: winnt (stream-seek) ( n stream -- ) 2drop ;
+
 : file-error? ( n -- eof? )
     zero? [
         GetLastError {

basis/io/ports/ports.factor

@@ -4,7 +4,7 @@ USING: math kernel io sequences io.buffers io.timeouts generic
 byte-vectors system io.encodings math.order io.backend
 continuations classes byte-arrays namespaces splitting
 grouping dlists assocs io.encodings.binary summary accessors
-destructors combinators unix ;
+destructors combinators ;
 IN: io.ports
 
 SYMBOL: default-buffer-size