gio: Add private API to create win32 streams from fds

This will be used by GSubprocess.

https://bugzilla.gnome.org/show_bug.cgi?id=672102
gspawn: support creating pipes with O_CLOEXEC

Add a new flag, G_SPAWN_CLOEXEC_PIPES, for creating the stdin/out/err
pipes with O_CLOEXEC (for the usual reasons).
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
gsubprocess: Fix up communicate

We weren't closing the streams after we were done reading or writing,
which is kind of essential.  The easy way to fix this is to just use
g_output_stream_splice() to a GMemoryOutputStream rather than
hand-rolling it.  This results in a substantial reduction of code
complexity.

A second serious issue is that we were marking the task as complete when
the process exits, but that's racy - there could still be data to read
from stdout.  Fix this by just refcounting outstanding operations.

This code, not surprisingly, looks a lot like the "multi" test.

Next, because processes output binary data, I'd be forced to annotate
the char*/length pairs as (array) (element-type uint8).  But rather than
doing that, it's *far* simpler to just use GBytes.

We need a version of this that actually validates as UTF-8, that will be
in the next patch.
gsubprocess: Add UTF-8 variants of communicate()

Over many years of writing code interacting with subprocesses, a pattern
that comes up a lot is to run a child and get its output as UTF-8, to
put inside a JSON document or render in a GtkTextBuffer, etc.

It's very important to validate at the boundaries, and not say deep
inside Pango.

We could do this a bit more efficiently if done in a streaming fashion,
but realistically this should be OK for now.
Fixup GSubprocess documentation bits