Sub-directories inside gio/ already processed in a previous commit:
- fam/
- gdbus-2.0/ (which contains only codegen/)
- gvdb/
- inotify/
- tests/
- win32/
- xdgmime/

Other sub-directories inside gio/:
- completion/: no license headers
- kqueue/: not LGPL, BSD-style license

https://bugzilla.gnome.org/show_bug.cgi?id=776504

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.

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.

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