Leave pos as undefined to allow reads from unseekable files like /proc/x/smaps on Linux

lib/fs.js

@@ -63,7 +63,8 @@ fs.readFile = function (path, encoding_, callback) {
     if (err) {
       if (callback) callback(err);
     } else {
-      readAll(fd, 0, "", encoding, callback);
+      // leave pos undefined to allow reads on unseekable devices
+      readAll(fd, undefined, "", encoding, callback);
     }
   });
 };
@@ -73,7 +74,7 @@ fs.readFileSync = function (path, encoding) {
 
   var fd = binding.open(path, process.O_RDONLY, 0666);
   var content = '';
-  var pos = 0;
+  var pos;   // leave undefined to allow reads on unseekable devices
   var r;
 
   while ((r = binding.read(fd, 4*1024, pos, encoding)) && r[0]) {