ostree-remount: mount a tmpfs on /var if necessary so /var is read-write

/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