child_process: give forks stdout and stderr

Previously using these streams was not possible because of making them
use given file descriptors. This solution is more generic - it uses
`pipe` function to pipe fork's output into parent's `stdout` and
`stderr` unless `silent: true` is set as an option.

This doesn't break any existing API and it makes `fork` more compatible
with `spawn`.

Please note that fork's `stdout` and `stderr` aren't meant to be used as
communication channels - use `fork(...).send()` instead.

Fixes nodejs#2442.

lib/child_process.js

@@ -167,10 +167,6 @@ exports.fork = function(modulePath, args, options) {
     throw new Error('customFds not allowed for fork()');
   }
 
-  // Leave stdin open for the IPC channel. stdout and stderr should be the
-  // same as the parent's if silent isn't set.
-  options.customFds = (options.silent ? [-1, -1, -1] : [-1, 1, 2]);
-
   // Just need to set this - child process won't actually use the fd.
   // For backwards compat - this can be changed to 'NODE_CHANNEL' before v0.6.
   if (!options.env) options.env = { };
@@ -181,6 +177,11 @@ exports.fork = function(modulePath, args, options) {
 
   var child = spawn(process.execPath, args, options);
 
+  if (!options.silent) {
+    child.stdout.pipe(process.stdout, { end: false });
+    child.stderr.pipe(process.stderr, { end: false });
+  }
+
   setupChannel(child, options.stdinStream);
 
   child.on('exit', function() {