update TODO
tmpfiles: always recreate the most basic directory structure in /var

Let's allow booting up with /var empty. Only create the most basic
directories to get to a working directory structure and symlink set in
/var.
nspawn: add new --tmpfs= option to mount a tmpfs on specific directories, such as /var
tmpfiles: static variables populated immediately from the command line should be prefixed with arg_
tmpfiles: when processing lines, always process prefixes before suffixes

If two lines refer to paths that are suffix and prefix of each other,
then always process the prefix first, the suffix second. In all other
cases strictly process rules in the order they appear in the files.

This makes creating /var/run as symlink to /run a lot more fun, since it
is automatically created first.