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"/>

docs/reference/gio/gio-sections.txt

@@ -4077,4 +4077,37 @@ G_SIMPLE_PROXY_RESOLVER_CLASS
 G_IS_SIMPLE_PROXY_RESOLVER_CLASS
 G_SIMPLE_PROXY_RESOLVER_GET_CLASS
 g_simple_proxy_resolver_get_type
+
+<FILE>gsubprocess</FILE>
+<TITLE>GSubprocess</TITLE>
+GSubprocess
+g_subprocess_new
+g_subprocess_newv
+<SUBSECTION IO>
+g_subprocess_get_stdin_pipe
+g_subprocess_get_stdout_pipe
+g_subprocess_get_stderr_pipe
+<SUBSECTION Waiting>
+g_subprocess_wait
+g_subprocess_wait_sync
+g_subprocess_wait_finish
+g_subprocess_wait_check
+g_subprocess_wait_check_sync
+g_subprocess_wait_check_finish
+<SUBSECTION Status>
+g_subprocess_get_successful
+g_subprocess_get_if_exited
+g_subprocess_get_exit_status
+g_subprocess_get_if_signaled
+g_subprocess_get_term_sig
+g_subprocess_get_status
+<SUBSECTION Control>
+g_subprocess_send_signal
+g_subprocess_force_exit
+<SUBSECTION Standard>
+G_IS_SUBPROCESS
+G_TYPE_SUBPROCESS
+G_SUBPROCESS
+<SUBSECTION Private>
+g_subprocess_get_type
 </SECTION>

docs/reference/gio/gio.types

@@ -137,3 +137,4 @@ g_test_dbus_get_type
 g_test_dbus_flags_get_type
 g_task_get_type
 g_simple_proxy_resolver_get_type
+g_subprocess_get_type