mount: use /dev/.run as an early boot alias for /var/run
During early boot, mount a tmpfs to /dev/.run and then bind mount it to /var/run as soon as /var is available. This makes it possible for programs involved in early boot to put runtime data in /dev/.run which later on will show up in /var/run like any other. This can be used to solve the early-boot D-Bus problem: D-Bus may start up with its socket bound to /dev/.run/dbus/system_bus_socket and after /var it will also be available under the traditional name /var/run/dbus/system_bus_socket. This also is intended to be used as a better place for systemd, mount, mdadm, blkid, plymouth, bootchart and dracut runtime data, which is currently stored in various places in /dev/.xxx.