There are a number of nice things this class brings:
0) On Unix, if WNOWAIT is available, has race-free termination API (we
don't reap the child until the GSubprocess is finalized, so the
GPid is always valid)
1) Operates in terms of G{Input,Output}Stream, not file descriptors
2) Async API instead of GSource
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
