Fix a couple of other bugs with the fs rewrite

- Fix stdin not being considered a "readable" input
 - Return an unsigned byte rather than a signed one for no-args .read()

src/main/java/dan200/computercraft/core/apis/handles/BinaryReadableHandle.java

@@ -118,7 +118,7 @@ public class BinaryReadableHandle extends HandleGeneric
                     {
                         single.clear();
                         int b = m_reader.read( single );
-                        return b == -1 ? null : new Object[] { single.get( 0 ) };
+                        return b == -1 ? null : new Object[] { single.get( 0 ) & 0xFF };
                     }
                 }
                 catch( IOException e )

src/main/resources/assets/computercraft/lua/rom/apis/io.lua

@@ -63,7 +63,7 @@ handleMetatable = {
             if self._closed then error("attempt to use a closed file", 2) end
 
             local handle = self._handle
-            if not handle.read then return nil, "Not opened for reading" end
+            if not handle.read and not handle.readLine then return nil, "Not opened for reading" end
 
             local n = select('#', ...)
             local output = {}