Merge waitpid() from g_spawn_sync into gmain()

This is preparatory work for a future commit which will add a
"catchall" waitpid API.  If we don't synchronize here with the worker
thread, race conditions are possible.

This also ensures we have an error message if someone adds a child
watch for a nonexistent pid, etc.  Previously, we'd simply keep
calling waitpid() getting ECHILD, and ignoring it until the source was
removed. Now, we g_warning() and fire the source.

Thirdly, this ensures that the waitpid() call in gmain handles EINTR,
like the g_spawn_sync() one did.

https://bugzilla.gnome.org/show_bug.cgi?id=687061