Merge pull request #2238 from oderwat/fix-2228-readall-osx

Fix for readAll(stdin) problem on OS X and better anyway. I can confirm this works on Windows.

lib/system/sysio.nim

@@ -145,8 +145,8 @@ proc readAllFile(file: File): string =
 proc readAll(file: File): TaintedString = 
   # Separate handling needed because we need to buffer when we
   # don't know the overall length of the File.
-  var len = rawFileSize(file)
-  if len >= 0:
+  let len = if file != stdin: rawFileSize(file) else: -1
+  if len > 0:
     result = readAllFile(file, len).TaintedString
   else:
     result = readAllBuffer(file).TaintedString