fix(systemd-rest+docker): Use keepdir to install empty /var directories.

Not only is this the safer way of installing empty directories but we
can now leverage keepdir to repopulate missing items in var.
bump(coreos-base/coreos-init): New code

Adds tmpfiles config, more units, and top level coreos-startup.target
which eliminates the need for maintaining that list in the ebuild.
fix(coreos-base/coreos-base): Remove dummy rootfs entry from fstab.

Remounting / read-write has been moved to the dev_mode helper script in
coreos-init instead of letting systemd do it based on fstab. Now if a
system boots without /root/.dev_mode the filesystem remains read-only.