FHS-3.0 says that /var/tmp should survive reboots, but for it is common
practice to ignore FHS for security reasons and wipe dirs that are world
writable.

There is no good reason to store nginx data under a world writable
directory, so move it back to /var/lib/nginx/tmp. Other distros does
something similar.

fixes #9246
fixes #10258
ref #9364

This reverts commit d6d624a1.