In general libglnx has expanded a lot to have a good set of low-level wrappers
for things like writing a buffer to a fd. Also, we should use `O_TMPFILE`
if available - I think the code reduction speaks for itself here.
Writing this patch as a result of looking at what fds flatpak injects.
However, *really* we want to use sealed memfds. I'll likely copy the
systemd wrappers for that into libglnx too.
Also, it took me a while to figure out the reason the `--args` code
worked before was because we were leaking the fd.
(Updated by Alexander Larsson <alexl@redhat.com> to use O_TMPFILE
in more places, like for the seccomp code, and rebased on
some preparatory cleanups)
Closes: #1060
Approved by: alexlarsson
0b32f63421