Use Process.spawn to spawn webkit_server whenever possible.

This allows one to close all unnecessary file descriptors in the child
process so they don't conflict with whatever the app is expecting.
It solves some problems with DRb.

lib/capybara/driver/webkit/browser.rb

@@ -93,10 +93,18 @@ def fork_server
       server_path = File.expand_path("../../../../../bin/webkit_server", __FILE__)
 
       read_pipe, write_pipe = IO.pipe
-      @pid = fork do
-        $stdout.reopen write_pipe
-        read_pipe.close
-        exec(server_path)
+      if Process.respond_to?(:spawn)
+        @pid = Process.spawn(server_path,
+          :in  => :in,
+          :out => write_pipe,
+          :err => :err,
+          :close_others => true)
+      else
+        @pid = fork do
+          $stdout.reopen write_pipe
+          read_pipe.close
+          exec(server_path)
+        end
       end
       at_exit { Process.kill("INT", @pid) }