remove DEBUG_FD

Now simply uses `process.stderr`. Breaking API change,
for the v3 branch.

Previously used internal and undocumented Node.js APIs to support
this underly used API.

Fixes #280
Closes #386

src/node.js

@@ -51,26 +51,14 @@ exports.inspectOpts = Object.keys(process.env).filter(function (key) {
   return obj;
 }, {});
 
-/**
- * The file descriptor to write the `debug()` calls to.
- * Set the `DEBUG_FD` env variable to override with another value. i.e.:
- *
- *   $ DEBUG_FD=3 node script.js 3>debug.log
- */
-
-var fd = parseInt(process.env.DEBUG_FD, 10) || 2;
-var stream = 1 === fd ? process.stdout :
-             2 === fd ? process.stderr :
-             createWritableStdioStream(fd);
-
 /**
  * Is stdout a TTY? Colored output is enabled when `true`.
  */
 
 function useColors() {
   return 'colors' in exports.inspectOpts
     ? Boolean(exports.inspectOpts.colors)
-    : tty.isatty(fd);
+    : tty.isatty(process.stderr.fd);
 }
 
 /**
@@ -115,11 +103,11 @@ function formatArgs(args) {
 }
 
 /**
- * Invokes `util.format()` with the specified arguments and writes to `stream`.
+ * Invokes `util.format()` with the specified arguments and writes to stderr.
  */
 
 function log() {
-  return stream.write(util.format.apply(util, arguments) + '\n');
+  return process.stderr.write(util.format.apply(util, arguments) + '\n');
 }
 
 /**
@@ -150,74 +138,6 @@ function load() {
   return process.env.DEBUG;
 }
 
-/**
- * Copied from `node/src/node.js`.
- *
- * XXX: It's lame that node doesn't expose this API out-of-the-box. It also
- * relies on the undocumented `tty_wrap.guessHandleType()` which is also lame.
- */
-
-function createWritableStdioStream (fd) {
-  var stream;
-  var tty_wrap = process.binding('tty_wrap');
-
-  // Note stream._type is used for test-module-load-list.js
-
-  switch (tty_wrap.guessHandleType(fd)) {
-    case 'TTY':
-      stream = new tty.WriteStream(fd);
-      stream._type = 'tty';
-
-      // Hack to have stream not keep the event loop alive.
-      // See https://github.com/joyent/node/issues/1726
-      if (stream._handle && stream._handle.unref) {
-        stream._handle.unref();
-      }
-      break;
-
-    case 'FILE':
-      var fs = require('fs');
-      stream = new fs.SyncWriteStream(fd, { autoClose: false });
-      stream._type = 'fs';
-      break;
-
-    case 'PIPE':
-    case 'TCP':
-      var net = require('net');
-      stream = new net.Socket({
-        fd: fd,
-        readable: false,
-        writable: true
-      });
-
-      // FIXME Should probably have an option in net.Socket to create a
-      // stream from an existing fd which is writable only. But for now
-      // we'll just add this hack and set the `readable` member to false.
-      // Test: ./node test/fixtures/echo.js < /etc/passwd
-      stream.readable = false;
-      stream.read = null;
-      stream._type = 'pipe';
-
-      // FIXME Hack to have stream not keep the event loop alive.
-      // See https://github.com/joyent/node/issues/1726
-      if (stream._handle && stream._handle.unref) {
-        stream._handle.unref();
-      }
-      break;
-
-    default:
-      // Probably an error on in uv_guess_handle()
-      throw new Error('Implement me. Unknown stream file type!');
-  }
-
-  // For supporting legacy API we put the FD here.
-  stream.fd = fd;
-
-  stream._isStdio = true;
-
-  return stream;
-}
-
 /**
  * Init logic for `debug` instances.
  *