GSubprocess: New class for spawning child processes

There are a number of nice things this class brings:

0) Has a race-free termination API on all platforms (on UNIX, calls to
   kill() and waitpid() are coordinated as not to cause problems).
1) Operates in terms of G{Input,Output}Stream, not file descriptors
2) Standard GIO-style async API for wait() with cancellation
3) Makes some simple cases easy, like synchronously spawning a
   process with an argument list
4) Makes hard cases possible, like asynchronously running a process
   with stdout/stderr merged, output directly to a file path

Much rewriting and code review from Ryan Lortie <desrt@desrt.ca>

https://bugzilla.gnome.org/show_bug.cgi?id=672102

gio/gsubprocesslauncher-private.h

@@ -0,0 +1,61 @@
+/* GIO - GLib Input, Output and Streaming Library
+ * 
+ * Copyright (C) 2012 Colin Walters <walters@verbum.org>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General
+ * Public License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place, Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#ifndef __G_SUBPROCESS_CONTEXT_PRIVATE_H__
+#define __G_SUBPROCESS_CONTEXT_PRIVATE_H__
+
+#include "gsubprocesslauncher.h"
+
+G_BEGIN_DECLS
+
+struct _GSubprocessLauncher
+{
+  GObject parent;
+
+  GSubprocessFlags flags;
+  gboolean path_from_envp;
+  char **envp;
+  char *cwd;
+
+#ifdef G_OS_UNIX
+  gint stdin_fd;
+  gchar *stdin_path;
+
+  gint stdout_fd;
+  gchar *stdout_path;
+
+  gint stderr_fd;
+  gchar *stderr_path;
+
+  GArray *basic_fd_assignments;
+  GArray *needdup_fd_assignments;
+
+  GSpawnChildSetupFunc child_setup_func;
+  gpointer child_setup_user_data;
+  GDestroyNotify child_setup_destroy_notify;
+#endif
+};
+
+void g_subprocess_set_launcher (GSubprocess         *subprocess,
+                                GSubprocessLauncher *launcher);
+
+G_END_DECLS
+
+#endif