/var needs to be read-write for a functioning system. Various
systemd services will fail to start if /var is read-only. After we
remount /var (or if we skip remounting /var because / is read-only),
if /var is still readonly, mount a tmpfs file system on /var.
While this isn't strictly part of ostree, keeping it here makes sense
because it keeps twiddling around with the /var mount in one place
for easier coordination and maintenance. This will likely need updating
if systemd gains better support for a readonly root filesystem.
https://bugzilla.gnome.org/show_bug.cgi?id=732979
The option --port-file=- is most useful when the stdout of the daemon
is programatically redirected and not going to a terminal. The
flush-after-a-line behavior of stdout is specific to terminals, so
we need an explicit flush.
https://bugzilla.gnome.org/show_bug.cgi?id=729609
Owen W. Taylor