Avoid the 3-second 'afterlife' of instances by using
kill (pid, 0) instead of checking the file lock. We
trade pid for lock races, but this seems more reliable.
Also add flatpak_installation_launch_full that returns
a FlatpakInstance for the launched app, and lets us
avoid races around killing the app, by using a child
watch.
Closes: #2221
Approved by: matthiasclasen
There is a race; flatpak_installation_launch may return
before bubblewrap has written the bwrapinfo.json file.
Make flatpak_instance_get_child_pid() deal with this and
document that the function may return 0 for a little
while.
This is made to let gnome-software enumerate all
running instances and get the same information
about them that flatpak ps provides.
For now, we keep it private. It will become library
api once we open new development
