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

docs/reference/gio/gio-docs.xml

@@ -108,6 +108,10 @@
         <xi:include href="xml/ginitable.xml"/>
         <xi:include href="xml/gasyncinitable.xml"/>
     </chapter>
+    <chapter id="subprocesses">
+      <title>Subprocesses</title>
+      <xi:include href="xml/gsubprocess.xml"/>
+    </chapter>
     <chapter id="networking">
       <title>Low-level network support</title>
       <xi:include href="xml/gsocket.xml"/>