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/giotypes.h

@@ -137,6 +137,7 @@ typedef struct _GIOStream                     GIOStream;
 typedef struct _GPollableInputStream          GPollableInputStream; /* Dummy typedef */
 typedef struct _GPollableOutputStream         GPollableOutputStream; /* Dummy typedef */
 typedef struct _GResolver                     GResolver;
+
 /**
  * GResource:
  *
@@ -513,6 +514,23 @@ typedef GType (*GDBusProxyTypeFunc) (GDBusObjectManagerClient   *manager,
 
 typedef struct _GTestDBus GTestDBus;
 
+/**
+ * GSubprocess:
+ *
+ * A child process.
+ *
+ * Since: 2.36
+ */
+typedef struct _GSubprocess                   GSubprocess;
+/**
+ * GSubprocessLauncher:
+ *
+ * Options for launching a child process.
+ *
+ * Since: 2.36
+ */
+typedef struct _GSubprocessLauncher           GSubprocessLauncher;
+
 G_END_DECLS
 
 #endif /* __GIO_TYPES_H__ */